PROGRAMMED CELL DEATH GENES AND PROTEINS

Abstract: This invention relates to genes involved in regulating programmed cell death, the proteins encoded by such genes and methods for controlling programmed cell death by regulating the activity of the cell death gene products.

Description: PROGRAMMED CELLDEATH GENESANDPROTEINS Background of the Invention Statement as to Rights to Inventions Made Under Federally-Sponsored Research and Development Part of the work performed during development of this invention utilized U.S. Government funds. The U.S. Government has certain rights in this invention. Cross-Reference to Related Applications This application is a continuation-in-part application of U.S. Application No. 08/080,580, filed June 24, . 1993. Field of the Invention The invention is in the field of molecular biology as related to the control of programmed cell death. Description of the Background Art Cell death occurs as a normal aspect of animal development as well as in tissue homeostasis and aging (Glucksmann, A., Biol. Rev. Cambridge Philos. Soc. 26:59-86 (1950); Ellis et al., Dev. 772:591-603 (1991)). Naturally occurring cell death acts to regulate cell number, to facilitate morphogenesis, to remove harmful or otherwise abnormal cells and to eliminate cells that have already performed their function. Such regulated cell death is achieved through a cell-endogenous mechanism of suicide, termed programmed cell death or apoptosis (Wyllie, A. H., in Cell Death in Biology and Pathology, Bowen and Lockshin, eds., Chapman and Hall (1981), pp. 9-34). Programmed cell death or apoptosis occurs when a cell activates this internally encoded suicide program as a result of either internal or external signals. The moφhological characteristics of apoptosis include plasma membrane blebbing, condensation of nucleoplasm and cytoplasm and degradation of chromosomal DNA at inter-nucleosomal intervals. (Wyllie, A. H., in Cell Death in Biology and Pathology, Bowen and Lockshin, eds., Chapman and Hall (1981), pp. 9-34). In many cases, gene expression appears to be required for programmed cell death, since death can be prevented by inhibitors of RNA or protein synthesis (Cohen et al., J. Immunol. 52:38-42 (1984); Stanisic et al., Invest. Urol. 76: 19-22 (1978); Martin et al., J. Cell Biol. 706:829-844 (1988)). The genetic control of programmed cell death has been well-elucidated by the work on programmed cell death in the nematode C. elegans. Programmed cell death is characteristic and widespread during C. elegans development. Of the 1090 somatic cells formed during the development of the hermaphrodite, 131 undergo programmed cell death. When observed with Nomarski microscopy, the moφhological changes of these dying cells follow a characteristic sequence. (Sulston et al., Dev. Biol. 82: 110-156 (1977); Sulston et al., Dev. Biol. 700:64-119 (1983)). Fourteen genes have been identified that function in different steps of the genetic pathway of programmed cell death in this nematode (Hedgecock et al., Science 220: 1277- 1280 (1983); Ellis et al., Cell 44:817-829 (1986); Ellis et al., Dev. 772:591- 603 (1991); Ellis et al., Genetics 772:591-603 (1991b); Hengartner et al., Nature 556:494-499 (1992); Ellis et al, Dev. 772:591-603 (1991)). Two of these genes, ced-3 and ced-4, play essential roles in either the initiation or execution of the cell death program. Recessive mutations in these genes prevent almost all of the cell deaths that normally occur during C. elegans development. Additional support for the view that ced-3 and ced-4 cause cell death comes from the genetic analysis of mosaics (Yuan et al, Dev. Biol. 758:33-41 (1990)). The ced-4 gene encodes a novel protein that is expressed primarily during embryogenesis, the period during which most programmed cell deaths occur (Yuan et al, Dev. 776:309-320 (1992)). A gain-of-function mutation in ced-9 prevents the normal programmed cell death, while mutations that inactivate ced-9 are lethal, suggesting that ced-9 may act by suppressing programmed cell death genes in cells that normally do not undergo programmed cell death (Hengartner, M., et al. , Nature 556:494-499 (1992)). The ced-9 gene encodes a protein product that shares sequence similarity with the mammalian proto-oncogene and cell death suppressor bcl-2 (Hengartner, M., et al , Cell 76:665-676 (1994)). The lethality of ced-9 loss-of-function mutations can be suppressed by mutations in ced-3 and ced-4, indicating that ced-9 acts by suppressing the activity of ced-3 and ced-4. Genetic mosaic analyses indicate that ced-3 and ced-4 likely act in a cell-autonomous fashion within dying cells, suggesting that they might be cytotoxic proteins and/or control certain cytotoxic proteins in the process of programmed cell death (Yuan, J., et aL , Dev. Bio. 138: 33-41 ( 1990)) . The 549 amino acid sequence of the ced-4 protein, deduced from cDNA and genomic clones, contain two regions that are similar to the calcium-binding domain known as the EF-hand (Kretsinger, 1987); however, it is still not clear at present whether calcium plays a role in regulating ced-4 or programmed cell death in C. elegans. Summary of the Invention In the present invention, the ced-3 gene has been cloned and sequenced and the amino acid sequence of the protein encoded by this gene is disclosed. Structural analysis of the ced-3 gene revealed that it is similar to the enzyme interleukin-1/3 converting enzyme ("ICE") and that overexpression of the murine interleukin-1/3 converting enzyme ("mICE") causes programmed cell death in vertebrate cells. Based upon these results, a novel method for controlling programmed cell death in vertebrates by regulating the activity of 7CE is claimed. The amino acid sequence of the ced-3 protein was also found to be similar to another murine protein, nedd-2, which is detected during early embryonic brain development, a period when many cells die. The results suggest that ced-3, mICE and nedd-2 are members of a gene family which function to cause programmed cell death. A new cell death gene, mICE2, has been discovered which appears to be in the same family as ced-3, mICE, and nedd-2. mICE2 is distinguished from other previously identified cell death genes in that it is preferentially expressed in the thymus and placental cells of vertebrates. Thus, the invention is also directed to a newly discovered gene, mICE2, which is preferentially expressed in thymus and placental cells and which encodes a protein causing programmed cell death. A comparison of the nucleotide sequences of ced-3, mICE, human 7CE, nedd-2 and mICE2 indicates that they are part of a gene family whose members all promote programmed cell death. The identification of this family facilitated the isolation of the newly discovered cell death gene Ice-ced 3 homolog (Ich-1). Ich-1 is homologous with the other cell death genes described above and particularly with nedd2. Based upon its structure and the presence of a QACRG sequence characteristic of the active center of cell death genes, Ich-1 was identified as a new member of the ced-3/ICE family. Thus, the present invention is directed to both the Ich-1 gene sequence and the Ich-1 protein. Also encompassed are vectors expressing Ich-1 and host cells transformed with such vectors. Alternative splicing results in two distinct Ich-1 mRNA species. Thus, the invention also encompasses these species, proteins produced from them, vectors containing and expressing the genes, and the uses described herein. The inventors have also identified a new member of the ICE/ced-3 family, Ice-4. Ice-4 has at least two alternative splicing products. A full length cDNA of one of them from a mouse thymus cDNA library has been identified. It encodes a protein of 418 amino acids that is 38% identical with murine ICE, 42% identical with murine 7ce-2, 25 % with murine Ich-1, and 24% identical with C. elegans ced-3. The invention is thus directed to genomic or cDNA nucleic acids having genetic sequences which encode ced-3, mICE2, Ich-1, and Ice-4. The invention also provides for vectors and expression vectors containing such genetic sequences, the host cells transformed with such vectors and expression vectors, the recombinant nucleic acid or proteins made in such host/vectors systems and the functional derivatives of these recombinant proteins. The use of the isolated genes or proteins for the puφose promoting cell death is also part of the invention. The invention is also directed to methods for controlling the programmed death of vertebrate cells by regulating the activity of interleukin- 1/3 converting enzyme, "ICE. " Such regulation may take the form of inhibiting the enzyme's activity, e.g. through the use of specific antiproteases such as crmA, in order to prevent cell death. In this way, it may be possible to develop cell lines which remain viable in culture for an extended period of time or indefinitely. Certain cells can only be maintained in culture if they are grown in the presence of growth factors. By blocking cell death, it may be possible to make such cells growth factor independent. Alternatively, 7CE activity may be increased in order to promote cell death. Such increased activity may be used in cancer cells to antagonize the effect of oncogenes such as bcl-2. Brief Description of the Figures Figure 1 and IA: Genetic and Physical Maps of the ced-3 Region on Chromosome TV Figure 1 shows the genetic map of C. elegans in the region near ced-3 with the cosmid clones representing this region depicted below the map. nP33, nP34, nP35, nP36, and nP37 are restriction fragment length polymoφhisms (RFLP) between Bristol and Bergerac wild type C. elegans strains. C43C9, W07H6 and C48D1 are three cosmid clones tested for rescue of the ced phenotype of ced-3(n717) animals. The ability of each cosmid clone to rescue ced-3 mutants and the fraction of independently obtained transgenic lines that were rescued are indicated on the right of the figure (+ , rescue; -, no rescue; see text for data). The results indicate that ced-3 is contained in the cosmid C48D1. Figure IA is a restriction map of C48D1 subclones. C48D1 was digested with BamHI and self-ligated to generate subclone C48D1-28. C48D1-43, pJ40 and pJ107 were generated by partial digesting C48D1-28 with Bglll. pJ7.5 and pJ7.4 were generated by JSCOIII deletion of pJ107. These subclones were assayed for rescue of the ced phenotype of ced-3(n717) animals (+ , rescue; -, no rescue, -/+ , weak rescue). The numbers in parentheses indicate the fraction of independently obtained transgenic lines that were rescued. The smallest fragment that fully rescued the ced-3 mutant phenotype was the 7.5 kb pJ7.5 subclone. Figure 2, 2A(i)-2A(v), 2B and 2C: Genomic Organization, Nucleotide Sequence, and Deduced Amino Acid Sequence of ced-3 Figure 2 shows the genomic sequence of the ced-3 region, as obtained from plasmid pJ107. The deduced amino acid sequence of the ced-3 protein is based on the DNA sequence of ced-3 cDNA pJ87 and upon other experiments described in the text and in Experimental Procedures. The 5' end of pJ87 contains 25 bp of poly-A/T sequence (not shown), which is probably a cloning artifact since it is not present in the genomic sequence. The likely start site of translation is marked with an arrowhead. The SL1 splice acceptor site of the ced-3 transcript is boxed. The positions of 12 ced-3 mutations are indicated. Repetitive elements are indicated as arrows above the relevant sequences. Numbers on the left indicate nucleotide positions, beginning with the start of pJ107. Numbers below the amino acid sequence indicate codon positions. Five types of imperfect repeats were found: repeat 1, also found in/em-7 (Spence et al, Cell 60:981-990 (1990)) and hlh-1 (Krause et al, Cell 65:907-919 (1990)); repeat 2, novel; repeat 3, also found in lin-12 and/em-7; repeat 4, also found in lin-12; and repeat 5, novel. Numbers on the sides of the figure indicate nucleotide positions, beginning with the start of pJ107. Numbers under the amino acid sequence indicate codon positions. Figure 2A(i) - Figure 2A(iv) contain comparisons of the repetitive elements in ced-3 with the repetitive elements in the genes ced-3, fem-1 , hlh-1 , lin-12, glp-1, and the cosmids B0303 and ZK643 (see text for references). In the case of inverted repeats, each arm of a repeat ("for" or "rev" for "forward" or "reverse", respectively) was compared to both its partner and to individual arms of the other repeats. 2A(i): Repeat 1; 2A(ii): Repeat 2; 2A(iii): Repeat 3; 2A(iv): Repeat 4; and 2A(v): Repeat 5. The different ced-3 sequences which appear in the comparisons are different repeats of the same repetitive element. The numbers "la", "lb" etc. are different repeats of the same class of repetitive element. Figure 2B shows the locations of the introns (lines) and exons (open boxes) of the ced-3 gene as well as the positions of 12 ced-3 mutations analyzed. The serine-rich region, the trans-spliced leader (SL1), the possible start of translation (ATG) and polyadenlyation (AAA) site are also indicated. Figure 2C shows the cDNA sequence and deduced amino acid sequence of ced-3 as obtained from plasmid pJ87. Figure 3 and 3A: Structure of the ced-3 Protein Figure 3 shows a comparison of structural features of ced-3 with those of the human interleukin-13 converting enzyme (ICE) gene. The predicted proteins corresponding to the ICE proenzyme and ced-3 are represented. The active site in 7CE and the predicted active site in ced-3 are indicated by the black rectangles. The four known cleavage sites in ICE flanking the processed 7CE subunits (p24, which was detected in low quantities when 7CE was purified (Thornberry et al , 1992), p20, and plO) and two conserved presumptive cleavage sites in the ced-3 protein are indicated with solid lines and linked with dotted lines. Five other potential cleavage sites in the ced-3 protein are indicated with dashed lines. The positions of the aspartate (D) residues at potential cleavage sites are indicated below each diagram. Figure 3A contains a comparison of the amino acid sequences of the ced-3 proteins from C. elegans, C. briggsae and C. vulgaris and the human and mouse 7CE and mouse nedd-2 proteins. Amino acids are numbered to the right of each protein. Dashes indicate gaps in the sequence made to allow optimal alignment. Residues that are conserved among more than half of the proteins are boxed. Missense ced-3 mutations are indicated above the comparison blocks showing the residue in the mutant ced-3 protein and the allele name. Asterisks indicate potential aspartate self-cleavage sites in the ced-3 protein. Circles indicate known aspartate self-cleavage sites in human 7CE. Residues indicated in boldface correspond to the highly conserved pentapeptide containing the active cysteine in 7CE. Figure 4: Construction of Expression Cassettes of mICE-lacZ and ced-3-lacZ Fusion Genes Figure 4 shows several expression cassettes used in studying the cellular effects of 7CE and ced-3 gene expression. The cassettes are as follows: pβactMIOZ contains intact mICE fused to the E. coli lacZ gene (mICE-lacZ). pβactMUZ contains the P20 and PlO subunits of mICE fused to the E. coli lacZ gene (P20/P10-/αcZ). p/3actM19Z contains the P20 subunit of mICE fused to the E. coli lacZ gene (P20-lacZ). pβactM12Z contains the PlO subunit of m7CE fused to the E. coli lacZ gene (P\0-lacZ). p/3actced38Z contains the C. elegans ced-3 gene fused to the lacZ gene (ced-3 '- lacZ) . pJ485 and pβactced37Z contain a Gly to Ser mutation at the active domain pentapeptide "QACRG" in mICE and ced-3 respectively. p/3actM17Z contains a Cys to Gly mutation at the active domain pentapeptide "QACRG" in mICE. pactøgal' is a control plasmid (Maekawa et al , Oncogene 6:627-632 (1991)). All plasmids use the /3-actin promoter. Figure 5: Genetic Pathways of Programmed Cell Death in the Nematode C. elegans and in Vertebrates In vertebrates, bcl-2 blocks the activity of 7CE thereby preventing programmed cell death. Εnzymatically active 7CE causes vertebrate cell death. In C. elegans, ced-9 blocks the action of ced-31 ced-4. Active ced-3 together with active ced-4 causes cell death. Figure 6: mICE2 cDNA Sequence and Deduced Amino Acid Sequence Figure 6 shows the nucleotide sequence of the mICE2 cDNA sequence and the amino acid sequence deduced therefrom. Figure 7 and 7 A: mICE2 Amino Acid Sequence Figures 7 and 7A contain a comparison of the amino acid sequences of murine interleukin-l converting enzyme (mICEl), human interleukin-lβ converting enzyme (hICE), mICE2 and ced-3. Figure 8: Ich-1 cDNA Sequence and Deduced Amino Acid Sequence Figure 8 shows the nucleotide sequence of the Ich-1 cDNA sequence and the amino acid sequence deduced therefrom. Figure 9: Potential QACRG Coding Region in the Mouse nedd2 cDNA The reading frame proposed by Kumar et al. (Biochem. & Biophys. Res. Comm. 185:1155-1161 (1992)) is b. In reading frame a, a potential QACRG coding region is underlined. Figure 10-lOC: Comparison of Mouse neddl and Ich-1 cDNA Sequences Figure 10-lOC contains a comparison of the mouse neddl cDNA sequence (top strand) and the Ich-1 cDNA sequence (bottom strand). The coding region for neddl starts at basepair 1177. Figure 11 and 11 A: Comparison of the Amino Acid Sequences of ced-3, ICE and Ich-1 Figure 11 contains a comparison of the amino acid sequences of ced-3 and Ich-1. There is a 52% similarity between the sequences and a 28% identity. Figure 1 IA contains a comparison of the amino acid sequences oi ICE and Ich-1. There is a 52% similarity between the sequences and a 27% identity. Figure 12A: The cDNA Sequence of Ich-1 L and the Deduced Amino Acid Sequence of Ich-1 L Protein Product The putative active domain is underlined. Figure 12B: The cDNA Sequence of Ich-l s and the Deduced Amino Acid Sequence of Ich-l s Protein Product The intron sequence is underlined. Figure 13: The Schematic Diagram of Ich-1 L and Ich-l s Figure 14: A comparison of the Ich-1 protein sequence with the mouse nedd-2 protein, the human interleukin-1 /..-converting enzyme (ICE) protein and C. elegans ced-3 protein. Amino acids are numbered to the right of each sequence. Any residues in nedd-2, ice and ced-3 that are identical with Ich-1 protein are highlighted. Figure 15: Stable Expression of Ich-l s Prevents Rat-1 Cells Induced by Serum Removal Stable transfectants of Rat-1 cells expressing bcl-2, crmA or Ich-l s were prepared as described in Experimental Procedures. Independent clones of both Ich-l s positive and Ich-l s negative were used. At time 0, exponentially growing cells were washed with serum-DMEM and dead cells were counted over time by trypan blue staining. Figure 16: The cDNA Sequence and Putative Ice-4 Protein Sequence The putative first Met is marked with a dot. Figure 17: Comparison of Amino Acid Sequences of Ice-4 with ICE, Ice-2, Ich-1 and ced-3 Definitions In the description that follows, a number of terms used in recombinant DNA (rDNA) technology or in the research area of programmed cell death are extensively utilized. In order to provide a clear and consistent understanding of the specification and claims, including the scope to be given such terms, the following definitions are provided. Gene. A DNA sequence containing a template for a RNA polymerase. The RNA transcribed from a gene may or may not code for a protein. RNA that codes for a protein is termed messenger RNA (mRNA). A "complementary DNA" or "cDNA" gene includes recombinant genes synthesized by reverse transcription of mRNA and from which intervening sequences (introns) have been removed. Cloning vector. A plasmid or phage DNA or other DNA sequence which is able to replicate autonomously in a host cell, and which is characterized by one or a small number of endonuclease recognition sites at which such DNA sequences may be cut in a determinable fashion without loss of an essential biological function of the vehicle, and into which DNA may be spliced in order to bring about its replication and cloning. The cloning vector may further contain a marker suitable for use in the identification of cells transformed with the cloning vehicle. Markers, for example, are tetracycline resistance or ampicillin resistance. The term "cloning vehicle" is sometimes used for "cloning vector. " Expression vector. A vector similar to a cloning vector but which is capable of expressing a gene which has been cloned into it, after transformation into a host. The cloned gene is usually placed under the control of (i.e., operably linked to) certain control sequences such as promoter sequences. Control sequences will vary depending on whether the vector is designed to express the operably linked gene in a prokaryotic or eukaryotic host and may additionally contain transcriptional elements such as enhancer elements, termination sequences, tissue-specificity elements, and/or translational initiation and termination sites. Programmed cell death. The process in which cell death is genetically programmed. Programmed cell death allows organisms to get rid of cells that have served a developmental puφose but which are no longer beneficial. Functional Derivative. A "functional derivative" of mICE2, Ich-1 (Ich-1 L and Ich-l s ), or Ice-4 is a protein which possesses a biological activity that is substantially similar to the biological activity of the non-recombinant. A functional derivative of may or may not contain post-translational modifications such as covalently linked carbohydrate, depending on the necessity of such modifications for the performance of a specific function. The term "functional derivative" is intended to include the "fragments," "variants," "analogues," or "chemical derivatives" of a molecule. Fragment. A "fragment" is meant to refer to any variant of the molecule, such as the peptide core, or a variant of the peptide core. Detailed Description of the Preferred Embodiments Description The present invention relates, inter alia, to isolated DNA encoding the ced-3 protein of C. elegans, mICE2, Ich-1, and Ice-4. The invention also encompasses nucleic acids having the cDNA sequence of ced-3, mICE-2, Ich-1, and Ice-4. The invention also encompasses related sequences in other species that can be isolated without undue experimentation. It will be appreciated that trivial variations in the claimed sequences and fragments derived from the full-length genomic and cDNA genes are encompassed by the invention as well. The invention also encompasses protein sequences from ced-3, Ich-1, and Ice-4. It should also be understood that by Ich-1 is intended both Ich-l s and lch-l L . ced-3 The genomic sequence of the claimed gene encoding ced-3 is shown in Figure 2. The gene is 7,656 base pairs in length and contains seven introns ranging in size from 54 base pairs to 1,195 base pairs. The four largest introns as well as sequences 5' to the START codon contain repetitive elements, some of which have been previously characterized in the non-coding regions of other C. elegans genes such as fem-1 (Spence et al , Cell 60:981- 990 (1990)) and hlh-I (Krause et al. , Cell 65:907-919 (1990)). A comparison of the repetitive elements in ced-3 with previously characterized repetitive elements is shown in figures 2A(i) - 2A(v). The START codon of the ced-3 protein is the methionine at position 2232 of the genomic sequence shown in Figure 2. The cDNA sequence of ced-3 shown in Figure 2C. The cDNA is 2,482 base pairs in length with an open reading frame encoding 503 amino acids and 953 base pairs of 3' untranslated sequence. The last 380 base pairs of the 3' sequence are not essential for the expression of the ced-3 protein. In addition to encompassing the genomic and cDNA sequences of ced-3 from C. elegans, the present invention also encompasses related sequences in other nematode species which can be isolated without undue experimentation. For example, the inventors have shown that ced-3 genes from C. briggsae and C. vulgaris may be isolated using the ced-3 cDNA from C. elegans as a probe (see Example 1). The invention also encompasses protein products from the ced-3 gene, gene variants, derivatives, and related sequences. As deduced from the DNA sequence, the ced-3 protein is 503 amino acids in length and contains a serine- rich middle region of about 100 amino acids. The amino acid sequence comprising the claimed ced-3 protein is shown in Figure 2 and Figure 2C. A comparison of the ced-3 protein of C. elegans with the inferred ced-3 protein sequences from the related nematode species C. briggsae and C. vulgaris indicates that the non-serine-rich region is highly conserved and that the serine-rich region is more variable. The non-serine-rich portion of the ced-3 protein is also homologous with interleukin-lj8 converting enzyme (ICE), a cysteine protease that can cleave the inactive 31 kD precursor of IL-13 to generate the active cytokine (Cerretti et al , Science 256:97-100 (1992); Thornberry et al , Nature 556:768-7 ' 4 (1992)). The C-terminal portions of both the ced-3 and 7CE proteins are similar to the mouse nedd-2 protein, which is encoded by an mRNA expressed during mouse embryonic brain development and down-regulated in the adult brain (Kumar et al. , Biochem. & Biophys. Res. Comm. 185: 1155-1161 (1992)). The results suggest that ced-3 acts as a cysteine protease in controlling the onset of programmed cell death in C. elegans and that members of the ced-3 IICEInedd-2 gene family function in programmed cell death in a wide variety of species. mICE-2 The cDNA sequence and deduced amino acid sequence of mICE2 are shown in Figure 6. As expected, mICE2 shows homology to both human and murine ICE as well as to C. elegans ced-3 (see Figure 7 and 7A). In contrast to other cell-death genes that have been identified, mICE2 is preferentially expressed in the thymus and placenta. Example 3 describes how the gene was obtained by screening a mouse thymus cDNA library with a DNA probe derived from human 7CE under conditions of low stringency. Given the amino acid sequence and cDNA sequence shown in Figure 6, preferred methods of obtaining the mICE2 gene (either genomic or cDNA) are described below. Ich-1 neddl, ICE, mICE2 and ced-3 are all members of the same gene family. This suggested that new genes might be isolated based upon their homology to identified family members. nedd2 is a mouse gene which is preferentially expressed during early embryonic brain development (Kumar et al, Biochem. Biophys. Res. Commun. 785: 1155-1161 (1992)). Since many neurons die during early embryonic brain development, it is possible that nedd-2 is a cell death gene. Ich-1 is 2492 base pairs in length and contains an open reading frame of 441 amino acids (Figure 8). The C-terminal 130 amino acids of Ich-1 are over 87% identical to mouse neddl. However, lch-1 contains a much longer open reading frame and has the pentapeptide QACRG which is the active center of the proteins of the ced-3/ICE family. The results indicate that the cDNA isolated by Kumar may not have been synthesized from a fully processed mRNA and that the 5' 1147 base pairs which Kumar reported for neddl cDNA may actually represent the sequence of an intron. The sequence reported by Kumar contains a region which could potentially code for QACRG but these amino acids are encoded in a different reading frame than that indicated by Kumar (Figure 9). This suggests that Kumar made an error in sequencing. The coding regions of neddl and n37 are highly homologous (Figure 10). The amino acid sequence of the deduced n37 protein shares 28 % identity with ced-3 and 27% identity with 7CE (Figure 11). The n37 protein was named Ich-1. Ich-1 mRNA is alternatively spliced into two different forms. One mRNA species encodes a protein product of 435 amino acids, designated Ich-1 L , which contains amino acid sequence homologous to both P20 and PlO subunits of 7CE as well as entire ced-3 protein. The other mRNA encodes a 312 amino-acid truncated version of Ich-1 protein, named Ich-l s , that terminates 21 amino acid residues after the QACRG active domain of Ich-1. Expression of Ich-1 L and Ich-l s has opposite effects on cell death. Overexpression of Ich-1 L induces Rat-1 fibroblast cells to die in culture, while overexpression of the Ich-l s suppresses Rat-1 cell death induced by serum deprivation. Results herein suggest that Ich-1 may play an important role in both positive and negative regulation of programmed cell death in vertebrate animals. Ice-4 Ice-4 was identified based on its sequence homology with 7CE and other isolated 7CE homologs. Since the lce-4 clone isolated by PCR only contains the coding region for the C-terminal half of the Ice-4 protein, a mouse thymus cDNA library was screened using the Ice-4 insert. Among 2 million clones screened, 9 positive clones were isolated. The sequence herein is from one clone that contains the complete coding region for Ice-4 gene. Methods of Making ced-3 There are many standard procedures for cloning genes which are well- known in the art and which can be used to obtain the ced-3 gene (see e.g., Sambrook et al. , Molecular Cloning, a Laboratory Manual, 2nd edition, vol. 1-3, Cold Spring Harbor Laboratory Press, 1989). In Example 1, a detailed description is provided of two preferred procedures. The first preferred procedure does not require the availability oi ced-3 gene sequence information and is based upon a method described by Ruvkun et al. (Molecular Genetics of Caenorhabditis Elegans Heterochromic Gene lin-14 121: 501-516 (1988)). In brief, Bristol and Bergerac strains of nematode are crossed and restriction fragment length polymoφhism mapping is performed on the DNA of the resulting inbred strain. Restriction fragments closely linked to ced-3 are identified and then used as probes to screen cosmid libraries for cosmids carrying all or part of the ced-3 gene. Positive cosmids are injected into a nematode strain in which ced-3 has been mutated. Cosmids carrying active ced-3 genes are identified by their ability to rescue the ced-3 mutant phenotype A second method for cloning ced-3 genes relies upon the sequence information which has been disclosed herein. Specifically, DNA probes are constructed based upon the sequence of the ced-3 gene of C. elegans. These probes are labelled and used to screen DNA libraries from nematodes or other species. Procedures for carrying out such cloning and screening are described more fully below in connection with the cloning and expression of mICE2, Ich-1, and Ice-4, and are well-known in the art (see, e.g., Sambrook et al , Molecular Cloning, a Laboratory Manual, 2nd edition (1988)). When hybridizations are carried out under conditions of high stringency, genes are identified which contain sequences corresponding exactly to that of the probe. In this way, the exact same sequence as described by the inventors herein may be obtained. Alternatively, hybridizations may be carried out under conditions of low stringency in order to identify genes in other species which are homologous to ced-3 but which contain structural variations (see Example 1 for a description of how such hybridizations may be used to obtain the ced-3 genes from C. briggsae and C. vulgaris). The results in Example 2 demonstrate that the products of cell-death genes may be tolerated by cells provided they are expressed at low levels. Therefore, the ced-3 protein may be obtained by incoφorating the ced-3 cDNA described above into any of a number of expression vectors well-known in the art and transferring these vectors into appropriate hosts (see Sambrook et al , Molecular Cloning, a Laboratory Manual, vol. 3 (1988)). As described below in connection with the expression of mICE2, Ich-1, and Ice-4, expression systems may be utilized in which cells are grown under conditions in which a recombinant gene is not expressed and, after cells reach a desired density, expression may be induced. In this way, the tendency of cells which express ced-3 to die may be circumvented. mICE2, Ich-1, and Ice-4 DNA encoding mICE2, Ich-1, and Ice-4 may be obtained from either genomic DNA or from cDNA. Genomic DNA may include naturally occurring introns. Moreover, such genomic DNA may be obtained in association wi the 5' promoter region of the sequences and/or with the 3' transcriptional termination region. Further, such genomic DNA may be obtained in association with the genetic sequences which encode the 5' non- translated region of the mICE2, Ich-1, and lce-4 mRNA and/or with the genetic sequences which encode the 3' non-translated region. To the extent that a host cell can recognize the transcriptional and/or translational regulatory signals associated with the expression of the mRNA and protein, then the 5' and/or 3' non-transcribed regions of the native gene, and/or, the 5' and/or 3' non-translated regions of the mRNA, may be retained and employed for transcriptional and translational regulation. Genomic DNA can be extracted and purified from any cell containing mouse chromosomes by means well known in the art (for example, see Guide to Molecular Cloning Techniques, S.L. Berger et al , eds., Academic Press (1987)). Alternatively, mRNA can be isolated from any cell which expresses the genes, and used to produce cDNA by means well known in the art (Id.). The preferred sources for mlCE2 are thymus or placental cells. The mRNA coding for any of the proteins (i.e., mICE2, Ich-1, or lce-4) may be enriched by techniques commonly used to enrich mRNA preparations for specific sequences, such as sucrose gradient centrifugation, or both. For cloning into a vector, DNA prepared as described above (either human genomic DNA or preferably cDNA) is randomly sheared or enzyma- tically cleaved, and ligated into appropriate vectors to form a recombinant gene library. A DNA sequence encoding the protein or its functional derivatives may be inserted into a DNA vector in accordance with conventional techniques. Techniques for such manipulations are disclosed by Sambrook, et al. , supra, and are well known in the art. In a preferred method, oligonucleotide probes specific for the gene are designed from the cDNA sequences shown in the Figures 6, 8, 12A, 12B, and 16. The oligonucleotide may be synthesized by means well known in the art (see, for example, Synthesis and Application of DNA and RNA, S.A. Narang, ed., 1987, Academic Press, San Diego, CA) and employed as a probe to identify and isolate the cloned gene by techniques known in the art. Techniques of nucleic acid hybridization and clone identification are disclosed by Maniatis, T., et al. (In: Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratories, Cold Spring Harbor, NY (1982)), and by Hames, B.D., etal. (In: Nucleic Acid Hybridization, A Practical Approach, IRL Press, Washington, DC (1985)). Those members of the above-described gene library which are found to be capable of such hybridization are then analyzed to determine the extent and nature of the coding sequences which they contain. To facilitate the detection of the desired coding sequence, the above- described DNA probe is labeled with a detectable group. This group can be any material having a detectable physical or chemical property. Such materials are well-known in the field of nucleic acid hybridization and any label useful in such methods can be applied to the present invention. Particularly useful are radioactive labels, such as 32 P, 3 H, 14 C, 35 S, 125 I, or the like. Any radioactive label may be employed which provides for an adequate signal and has a sufficient half-life. The oligonucleotide may be radioactively labeled, for example, by "nick-translation" by well-known means, as described in, for example, Rigby, P.J.W., et al , J. Mol. Biol. 775:237 (1977) or by T4 DNA polymerase replacement synthesis as described in, for example, Deen, K.C., et al , Anal. Biochem. 755:456 (1983). Alternatively, oligonucleotide probes may be labeled with a non- radioactive marker such as biotin, an enzyme or a fluorescent group. See, for example, Leary, J.J., et al , Proc. Natl. Acad. Sci. USA 80:4045 (1983); Renz, M., et al , Nucl. Acids Res. 72:3435 (1984); and Renz, M., EMBO J. 6:817 (1983). For Ich-1, the isolation shown in the Examples was as follows. Two primers were used in the polymerase chain reaction to amplify nedd2 cDNA from embryonic day 15 mouse brain cDNA (Sambrook et al., Molecular Cloning, a Laboratory Manual, vol. 3 (1988)). One primer had the sequence: ATGCTAACTGTCCAAGTCTA and the other primer had the sequence: TCCAACAGCAGGAATAGCA. The cDNA thus amplified was cloned using standard methodology. The cloned mouse neddl cDNA was used as a probe to screen a human fetal brain cDNA library purchased from Stratagene. Such methods of screening and isolating clones are well known in the art (Maniatis, T., et al , Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratories, Cold Spring Harbor, NY (1982)); Hames, B.D., et al, Nucleic Acid Hybridization, A Practical Approach, IRL Press, Washington, DC (1985)). A human nedd-2 cDNA clone was isolated that encodes a protein much longer than the mouse nedd-2 and contains amino acid sequences homologous to the entire 7CE and ced-3 proteins. The isolated clone was given the name [ce-ced 3 homolog or Ich-1. The Ich-1 cDNA may be obtained using the nucleic acid sequence information given in Figures 8, 12A, or 12B. DNA probes constructed from this sequence can be labeled and used to screen human gene libraries as described herein. Also as discussed herein, Ich-1 may be cloned into expression vectors and expressed in systems in which host cells are grown under conditions in which recombinant genes are not expressed and, after cells reach a desired density, expression is induced. In this way, a tendency of cells which express Ich-1 to die may be circumvented. One method of making Ice-4 is as follows. mRNA was isolated from embryonic day 14 mouse embryos using Invitrogens' microfast track mRNA isolation kit. The isolated mRNA was reverse transcribed to generate template for PCR amplification. The degenerate PCR primers were: c7ce-S {TG(ATCG)CC(ATCG)GGGAA(ATCG)AGGTAGAA} and cTceAs {ATCAT(ATC)ATCCAGGC(ATCG)TGCAG(AG)GG}. The PCR cycles were set up as follows: 1. 94°C, 3 min; 2. 94°C, 1 min; 3. 48°C, 2 min; 4. 72°C, 3 min; 5. return to "2" 4 cycles; 6. 94°C, 1 min; 7. 55°C, 2 min; 8. 72°C, 3 min; 9. return to "6" 34 cycles; 10. 72°C, 10 min; 11. end. Such PCR generated a band about 400bp, the predicted size of 7CE homologs. The PCR products were cloned into T-tailed blunt-ended pBSKII plasmid vector (Stratagene). Plasmids that contain an insert were analyzed by DNA sequencing. The Ice-4 cDNA may also be obtained using the nucleic acid sequence information given in Figure 16. DNA probes constructed from this sequence can be labeled and used to screen human gene libraries as described herein. Also as discussed herein, Ice-4 may be cloned into expression vectors and expressed in systems in which host cells are grown under conditions in which recombinant genes are not expressed and, after cells reach a desired density, expression is induced. The methods discussed herein are capable of identifying genetic sequences which encode mICE2, Ich-1, and Ice-4. In order to further charac¬ terize such genetic sequences, and, in order to produce the recombinant protein, it is desirable to express the proteins which these sequences encode. To express any of the genes herein (mICE2, Ich-1, Ice-4, and derivatives), transcriptional and translational signals recognizable by an appropriate host are necessary. The cloned coding sequences, obtained through the methods described herein, may be operably linked to sequences controlling transcriptional expression in an expression vector and introduced into a host cell, either prokaryote or eukaryote, to produce recombinant protein or a functional derivative thereof. Depending upon which strand of the sequence is operably linked to the sequences controlling transcriptional expression, it is also possible to express antisense RNA or a functional derivative thereof. Expression of the protein in different hosts may result in different post- translational modifications which may alter the properties of the protein. Preferably, the present invention encompasses the expression oimICE2, Ich-1, and Ice-4 or a functional derivative thereof, in eukaryotic cells, and especially mammalian, insect and yeast cells. Especially preferred eukaryotic hosts are mammalian cells either in vivo, or in tissue culture. Mammalian cells provide post-translational modifications which should be similar or identical to those found in the native protein. Preferred mammalian host cells include rat-1 fibroblasts, mouse bone marrow derived mast cells, mouse mast cells immortalized with Kirsten sarcoma virus, or normal mouse mast cells that have been co-cultured with mouse fibroblasts. Razin et al, J. of Immun. 752: 1479 (1984); Levi-Schaffer et al , Proc. Natl. Acad. Sci. (USA) 85:6485 (1986) and Reynolds et al , "Immortalization of Murine Connective Tissue- type Mast Cells at Multiple Stages of Their Differentiation by Coculture of Splenocytes with Fibroblasts that Produce Kirsten Sarcoma Virus," J. Biol. Chem. 265: 12783-12791 (1988). A nucleic acid molecule, such as DNA, is said to be "capable of expressing" a polypeptide if it contains expression control sequences which contain transcriptional regulatory information and such sequences are "operably linked" to the nucleotide sequence which encodes the polypeptide. An operable linkage is a linkage in which a coding sequence is connected to a regulatory sequence (or sequences) in such a way as to place expression of the coding sequence under the influence or control of the regulatory sequence. Two DNA sequences (e.g. the coding sequence of protein and a promoter) are said to be operably linked if induction of promoter function results in the transcription of the coding sequence and if the nature of the linkage between the two DNA sequences does not (1) result in the introduction of a frame-shift mutation; (2) interfere with the ability of regulatory sequences to direct the expression of the coding sequence, antisense RNA, or protein; or (3) interfere with the ability of the coding sequence template to be transcribed by the promoter region sequence. Thus, a promoter region would be operably linked to a DNA sequence if the promoter were capable of effecting transcription of that DNA sequence. The precise nature ofthe regulatory regions needed for gene expression may vary between species or cell types, but shall in general include, as necessary, 5' non-transcribing and 5' non-translating (non-coding) sequences involved with initiation of transcription and translation respectively, such as the TATA box, capping sequence, CAAT sequence, and the like. Especially, such 5' non-transcribing control sequences will include a region which contains a promoter for transcriptional control of the operably linked gene. Expression of proteins of the invention in eukaryotic hosts requires the use of regulatory regions functional in such hosts, and preferably eukaryotic regulatory systems. A wide variety of transcriptional and translational regu¬ latory sequences can be employed, depending upon the nature of the eukaryotic host. The transcriptional and translational regulatory signals can also be derived from the genomic sequences of viruses which infect eukaryotic cells, such as adenovirus, bovine papilloma virus, Simian virus, heφes virus, or the like. Preferably, these regulatory signals are associated with a particular gene which is capable of a high level of expression in the host cell. In eukaryotes, where transcription is not linked to translation, control regions may or may not provide an initiator methionine (AUG) codon, depending on whether the cloned sequence contains such a methionine. Such regions will, in general, include a promoter region sufficient to direct the initiation of RNA synthesis in the host cell. Promoters from heterologous mammalian genes which encode mRNA capable of translation are preferred, and especially, strong promoters such as the promoter for actin, collagen, myosin, etc., can be employed provided they also function as promoters in the host cell. Preferred eukaryotic promoters include the promoter of the mouse metallothionein I gene (Hamer, D., et al , J. Mol. Appl. Gen. 7:273-288 (1982)); the TK promoter of Heφes virus (McKnight, S., Cell 57:355-365 (1982)); the SV40 early promoter (Benoist, C, et al , Nature (London) 290:304-310 (1981)); in yeast, the yeast gal4 gene promoter (Johnston, S.A., et al , Proc. Natl Acad. Sci. (USA) 79:6971-6975 (1982); Silver, P. A., et al , Proc. Natl. Acad. Sci. (USA) 87:5951-5955 (1984)) or a glycolytic gene promoter may be used. It is known that translation of eukaryotic mRNA is initiated at the codon which encodes the first methionine. For this reason, it is preferable to ensure that the linkage between a eukaryotic promoter and a DNA sequence which encodes the proteins of the invention or functional derivatives thereof, does not contain any intervening codons which are capable of encoding a methionine. The presence of such codons results either in the formation of a fusion protein or a frame-shift mutation. If desired, a fusion product of the proteins may be constructed. For example, the sequence coding for the proteins may be linked to a signal sequence which will allow secretion of the protein from, or the compartmentalization of the protein in, a particular host. Such signal sequences may be designed with or without specific protease sites such that the signal peptide sequence is amenable to subsequent removal. Alternatively, the native signal sequence for this protein may be used. Transcriptional initiation regulatory signals can be selected which allow for repression or activation, so that expression of operably linked genes can be modulated. Of interest are regulatory signals which are temperature- sensitive so that by varying the temperature, expression can be repressed or initiated, or are subject to chemical regulation, e.g., metabolite. If desired, the non-transcribed and/or non-translated regions 3' to the sequence coding for the proteins can be obtained by the above-described cloning methods. The 3 '-non-transcribed region may be retained for transcriptional termination regulatory sequence elements; the 3 '-non- translated region may be retained for translational termination regulatory sequence elements, or for those elements which direct polyadenylation in eukaryotic cells. Where native expression control signals do not function satisfactorily in a host cell, functional sequences may be substituted. The vectors of the invention may further comprise other operably linked regulatory elements such as enhancer sequences, or DNA elements which confer tissue or cell-type specific expression on an operably linked gene. To transform a mammalian cell with the DNA constructs of the invention many vector systems are available, depending upon whether it is desired to insert the DNA construct into the host cell chromosomal DNA, or to allow it to exist in extrachromosomal form. If the protein encoding sequence and an operably linked promoter are introduced into a recipient eukaryotic cell as a non-replicating DNA (or RNA) molecule, the expression of the protein may occur through the transient expression of the introduced sequence. In a preferred embodiment, genetically stable transformants may be constructed with vector systems, or transformation systems, whereby mICE2, Ich-1, or Ice-4 DNA is integrated into the host chromosome. Such integration may occur de novo within the cell or, in a most preferred embodiment, through the aid of a cotransformed vector which functionally inserts itself into the host chromosome, for example, retroviral vectors, transposons or other DNA elements which promote integration of DNA sequences in chromosomes. Cells which have stably integrated the introduced DNA into their chromosomes are selected by also introducing one or more markers which allow for selection of host cells which contain the expression vector in the chromosome, for example the marker may provide biocide resistance, e.g., resistance to antibiotics, or heavy metals, such as copper, or the like. The selectable marker gene can either be directly linked to the DNA gene sequences to be expressed, or introduced into the same cell by co-transfection. In another embodiment, the introduced sequence is incoφorated into a plasmid or viral vector capable of autonomous replication in the recipient host. Any of a wide variety of vectors may be employed for this puφose. Factors of importance in selecting a particular plasmid or viral vector include: the ease with which recipient cells that contain the vector may be recognized and selected from those recipient cells which do not contain the vector; the number of copies of the vector which are desired in a particular host; and whether it is desirable to be able to "shuttle" the vector between host cells of different species. Preferred eukaryotic plasmids include those derived from the bovine papilloma virus, vaccinia virus, SV40, and, in yeast, plasmids containing the 2-micron circle, etc., or their derivatives. Such plasmids are well known in the art (Botstein, D., et al, Miami Wntr. Symp. 19:265-274 (1982); Broach, J.R., In: The Molecular Biology ofthe Yeast Saccharomyces: Life Cycle and Inheritance, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, p. 445-470 (1981); Broach, J.R., Cell 28:203-204 (1982); Bollon, D.P., et al , J. Clin. Hematol. Oncol 70:39-48 (1980); Maniatis, T., In: Cell Biology: A Comprehensive Treatise, Vol. 3, Gene Expression, Academic Press, NY, pp. 563-608 (1980)), and are commercially available. Once the vector or DNA sequence containing the construct(s) is prepared for expression, the DNA construct(s) is introduced into an appropriate host cell by any of a variety of suitable means, including transfection. After the introduction of the vector, recipient cells are grown in a medium which selects for the growth of vector-containing cells. Expression of the cloned gene sequence(s) results in the production of the protein, or in the production of a fragment of this protein. This expression can take place in a continuous manner in the transformed cells, or in a controlled manner, for example, expression which follows induction of differentiation of the transformed cells (for example, by administration of bromodeoxyuracil to neuroblastoma cells or the like). The latter is preferred for the expression of the proteins of the invention. By growing cells under conditions in which the proteins are not expressed, cell death may be avoided. When a high cell density is reached, expression of the proteins may be induced and the recombinant protein harvested immediately before death occurs. The expressed protein is isolated and purified in accordance with conventional procedures, such as extraction, precipitation, gel filtration chromatography, affinity chromatography, electrophoresis, or the like. The mICE2, Ich-1, and Ice-4 sequences, obtained through the methods above, will provide sequences which not only encode these proteins but which also encode antisense RNA directed against mICE2, Ich-1, and Ice-4; the antisense DNA sequence will be that sequence found on the opposite strand of the strand transcribing the mRNA. The antisense DNA strand may also be operably linked to a promoter in an expression vector such that transformation with this vector results in a host capable of expression of the antisense RNA in the transformed cell. Antisense DNA and RNA may be used to interact with endogenous mICE2, Ich-1, or Ice-4 DNA or RNA in a manner which inhibits or represses transcription or translation of the genes in a highly specific manner. Use of antisense nucleic acid to block gene expression is discussed in Lichtenstein, C, Nature 555:801-802 (1988). Methods of Using ced-3 The ced-3 gene (as well as ced-3 homologs and other members of the ced-3 gene family) may be used for a number of distinct puφoses. First, portions of the gene may be used as a probe for identifying genes homologous to ced-3 in other strains of nematode (see Example 1) as well as in other species (see Examples 2 and 3). Such probes may also be used to determine whether the ced-3 gene or homologs of ced-3 are being expressed in cells. The cell death genes will be used in the development of therapeutic methods for diseases and conditions characterized by cell death. Among diseases and conditions which could potentially be treated are neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. The discovery that ced-3 is related to ICE suggests that cell death genes may play an important role in inflammation (IL-ljS is known to be involved in the inflammatory response). Thus therapeutics based upon ced-3 and related cell death genes may also be developed. mICE2, Ich-1, and Ice-4 mICE2, Ich-1, and Ice-4 will have the same uses as those described in connection with ced-3 (above) and 7CE (see below). The gene sequences may be used to construct antisense DNA and RNA oligonucleotides, which, in turn, may be used to prevent programmed cell death in thymus or placental cells. Techniques for inhibiting the expression of genes using antisense DNA or RNA are well-known in the art (Lichtenstein, C. , Nature 555:801-802 (1988)). Portions of the claimed DNA sequence may also be used as probes for determining the level of expression. Similarly the protein may be used to generate antibodies that can be used in assaying cellular expression. Portions of the mICE2, Ich-1, and lce-4 genes described above may be used for determining the level of expression of the proteins (mICE2 in thymus or placental cells as well as in other tissues and organs). Such methods may be useful in determining if these cells have undergone a neoplastic transformation. Probes based upon the gene sequences may be used to isolate similar genes involved in cell death. A portion of the gene may be used in homologous recombination experiments to repair defective genes in cells or, alternatively, to develop strains of mice that are deficient in the gene. Antisense constructs may be transfected into cells (placental or thymus cells for mICEl) in order to develop cells which may be maintained in culture for an extended period of time or indefinitely. Alternatively antisense constructs may be used in cell culture or in vivo to block cell death. The protein may be used for the puφose of generating polyclonal or monoclonal antibodies using standard techniques well known in the art (Klein, J. Immunology: The Science of Cell-Noncell Discrimination, John Wiley & Sons, N.Y. (1982); Kennett et al, Monoclonal Antibodies, Hybridoma: A New Dimension in Biological Analyses, Plenum Press, N.Y. (1980); Campbell, A., "Monoclonal Antibody Technology," In: Laboratory Techniques in Biochemistry and Molecular Biology 13, Burdon et al. eds. , Elseiver, Amsterdam (1984); Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988)). Such antibodies may be used in assays for determining the expression of the genes. Purified protein would serve as the standard in such assays. Based upon the sequences of Figures 6, probes may be used to determine whether the mICE2 gene or homologs of mICE2 are being expressed in cells. Such probes may be utilized in assays for correlating mICE2 expression with cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to mICE2. mICE2 will be used in the development of therapeutic methods for diseases and conditions characterized by cell death. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. Antisense nucleic acids based upon the sequences shown in Figure 6 may be used to inhibit mICE2 expression. Such inhibition will be useful in blocking cell death in cultured cells. The mICE2 protein may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (Klein, J. Immunology: The Science of Cell-Noncell Discrimination, John Wiley & Sons, N.Y. (1982); Kennett et al. , Monoclonal Antibodies, Hybridoma: A New Dimension in Biological Analyses, Plenum Press, N.Y. (1980); Campbell, A., "Monoclonal Antibody Technology, " In: Laboratory Techniques in Biochemistry and Molecular Biology 13, Burdon et al. eds., Elseiver, Amsterdam (1984); Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988)). The antibodies may be used in assays for determining the expression of mICE2. Purified mICE2 protein would serve as the standard in such assays. Based upon the sequences of Figures 8, 12A, and 12B, probes may be used to determine whether the Ich-1 gene or homologs of Ich-1 are being expressed in cells. Such probes may be utilized in assays for correlating Ich-1 expression with cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to Ich-1. Ich-1 will be used in the development of therapeutic methods for diseases and conditions characterized by cell death. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. Antisense nucleic acids based upon the sequences shown in Figures 8, 12A, and 12B, may be used to inhibit Ich-1 expression. Such inhibition will be useful in blocking cell death in cultured cells. The Ich-1 protein may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (Klein, J. Immunology: The Science of Cell-Noncell Discrimination, John Wiley & Sons, N.Y. (1982); Kennett et al, Monoclonal Antibodies, Hybridoma: A New Dimension in Biological Analyses, Plenum Press, N.Y. (1980); Campbell, A., "Monoclonal Antibody Technology," In: Laboratory Techniques in Biochemistry and Molecular Biology 13, Burdon et al. eds., Elseiver, Amsterdam (1984); Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988)). The antibodies may be used in assays for determining the expression oilch-l. Purified Ich-1 protein would serve as the standard in such assays. Based upon the sequence of Figure 16, probes may be used to determine whether the lce-4 gene or homologs of Ice-4 are being expressed in cells. Such probes may be utilized in assays for correlating lce-4 expression with cellular conditions, e.g. neoplastic transformation, as well as for the puφose of isolating other genes which are homologous to lce-4. lce-4 will be used in the development of therapeutic methods for diseases and conditions characterized by cell death. The diseases and conditions which could potentially be treated include neural and muscular degenerative diseases, myocardial infarction, stroke, virally induced cell death and aging. Antisense nucleic acids based upon the sequence shown in Figure 16 may be used to inhibit lce-4 expression. Such inhibition will be useful in blocking cell death in cultured cells. The lce-4 protein may be used to generate polyclonal or monoclonal antibodies using methods well known in the art (Klein, J. Immunology: The Science of Cell-Noncell Discrimination, John Wiley & Sons, N.Y. (1982); Kennett et al, Monoclonal Antibodies, Hybridoma: A New Dimension in Biological Analyses, Plenum Press, N.Y. (1980); Campbell, A., "Monoclonal Antibody Technology," In: Laboratory Techniques in Biochemistry and Molecular Biology 13, Burdon et al eds., Elseiver, Amsterdam (1984); Harlow and Lane, Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y. (1988)). The antibodies may be used in assays for determining the expression of lce-4. Purified lce-4 protein would serve as the standard in such assays. Method for Preventing Programmed Cell Death in Vertebrate Cells by Inhibiting the Enzymatic Activity of Interleukin-lβ Converting Enzyme (ICE) The present invention is directed to preventing the programmed death of vertebrate cells by inhibiting the action of 7CE. The detailed structural analysis performed on the ced-3 gene from C. elegans revealed a homology to human and murine ICE which is especially strong at the QACRG active domain of ICE (see Figure 3A). 7CE is a cysteine protease that cleaves inactive pro-interleukin-0 into active interleukin-1/3. In order to determine if 7CE functions as a cell death gene in vertebrates, the mouse 7CE gene was cloned, inserted into an expression vector and then transfected into rat cells. A close correlation was found between 7CE expression and cell death (see Example 2). Further support for the function of 7CE as a cell death gene was obtained from inhibition studies. The cowpox gene crmA encodes a protein that specifically inhibits 7CE activity (Ray et al. , Cell 69:597-604 (1992)). In order to determine whether cell death can be prevented by inhibiting the enzymatic action of 7CE, cell lines were established which produced a high level of crmA protein. When these cells were transfected with 7CE, it was found that a large percentage of the cells expressing 7CE maintained a healthy moφhology and did not undergo programmed cell death. Evidence that 7CE has a physiological role as a vertebrate cell death gene was also obtained by examining cells engineered to over-express bcl-2, an oncogene known to inhibit programmed cell death and to be overexpressed in many follicular and B cell lymphomas. It was found that cells expressing bcl-2 did not undergo cell death despite the synthesis of high levels of 7CE. These results suggest that bcl-2 may promote malignancy by inhibiting the action of 7CE. Any method of specifically regulating the action of 7CE in order to control programmed cell death in vertebrates is encompassed by the present invention. This would include not only inhibitors specific to ICE, e.g. crmA, or the inhibitors described by Thornberry et al, Nature 556:768-774 (1992), but also any method which specifically prevented the expression of the ICE gene. Thus, antisense RNA or DNA comprised of nucleotide sequences complementary to 7CEand capable of inhibiting the transcription or translation of 7CE are within the scope of the invention (see Lichtenstein, C , Nature 555:801-802 (1988)). The ability to prevent vertebrate programmed cell death is of use in developing cells which can be maintained for an indefinite period of time in culture. For example, cells over-expressing crmA may be used as hosts for expressing recombinant proteins. The ability to prevent programmed cell death may allow cells to live independent of normally required growth factors. It has been found that microinjecting crmA mRNA or a crra/l-expressing nucleic acid construct into cells allows chicken sympathetic neurons to live in vitro after the removal of neural growth factor. Alternatively, the expression of 7CEmay be increased in order to cause programmed cell death. For example, homologous recombination may be used to replace a defective region of an 7CE gene with its normal counteφart. In this way, it may be possible to prevent the uncontrolled growth of certain malignant cells. Methods of increasing 7CE activity may be used to kill undesired organisms such as parasites. crmA is a viral protein which is important for cowpox infection. This suggests that the prevention of cell death may be important for successful infection and that, by the promotion of 7CE expression, may provide a means for blocking infection. Having now generally described this invention, the same will be further described by reference to certain specific examples which are provided herein for puφoses of illustration only and are not intended to be limiting unless otherwise specified. All references cited throughout the specification are incoφorated by reference in their entirety. Example 1 Experimental Procedures General Methods and Strains The techniques used for culturing C. elegans have been described by Brenner (Brenner, S., Genetics 77:71-94 (1974)). All strains were grown at 20°C. The wild-type parent strains were C. elegans variety Bristol strain N2, Bergerac strain EM1002 (Emmons et al, Cell 32:55-65 (1983)), C. briggsae and C. vulgaris. The genetic markers used are described below. These markers have been previously described (Brenner, S., Genetics 77:71-94 (1974)); and Hodgkin et al. , Genetics in the Nematode Caenorhabditis Elgens (Wood et al. eds.) pp.491-584, Cold Spring Harbor, New York (1988)). Genetic nomenclature follows the standard system (Horvitz et al, Mol. Gen. Genet. 775:129-133 (1979)). LG I: ced-1 (ei 735); unc-54 (r323) LG VI: unc-31 (e928), unc-30 (el91), ced-3 (n717, n718, nl040, nll29, nl!634, nll64, nl!65, nl286, nl949, n2426, n2430, n2433), unc-26 (e205), dpy-4 (ell66) LG V: eg-l(n986); unc-76 (e911) LG X: dpy-3(e27) Isolation of additional alleles of ced-3 A non-complementation screen was designed to isolate new alleles of ced-3. Because animals heterozygous for ced3(n717) in trans to a deficiency are viable (Ellis et al, Cell 44:817-829 (1986)), it was expected that animals carrying a complete loss-of- function mutant ced-3 allele in trans to ced-3 (n717) would be viable even if homozygotes for the allele were inviable. EMS mutagenized egl-1 L4 males were mated with ced-3(n7l7) unc-26(e205); egl-1 (n487); dpy-3(e27) hermaphrodites, egl-1 was used as a marker in this screen. Dominant mutations in egl-1 cause the two hermaphrodite-specific neurons, the HSNs, to undergo programmed cell death (Trent et al, Genetics 704:619-647 (1983)). The HSNs are required for normal egg-laying, and egl-1 (n986) hermaphrodites, which lack HSNs are egg-laying defective (Trent et al, Genetics 704:619-647)). The mutant phenotype of egl-1 is suppressed in a ced-3; egl-1 strain because mutations in ced-3 block programmed cell deaths, egl-1 males were mutagenized with EMS and crossed with ced3(n717) unc-26(e205); egl-1 (n487); dpy-3(e27). Most cross progeny were egg-laying defective because they were heterozygous for ced-3 and homozygous for egl-1. Rare egglaying competent animals were picked, those animals being candidates for carrying new alleles of ced-3. Four such animals were isolated from about 10,000 Fl cross progeny of EMS-mutagenized animals. These new mutations were made homozygous to confirm that they carried mutations of ced-3. RFLP mapping Two cosmid libraries were used extensively in this work - a Sau3A I partial digest genomic library of 7000 clones in the vector pHC79 and a Sau3A I partial digest genomic library of 6000 clones in the vector pJB8 (Coulson et al , Proc. Natl. Acad. Sci. U.S.A. 85:7821-7825 (1986). Bristol (N2) and Bergerac (EM 1002) DNA was digested with various restriction enzymes and probed with different cosmids to look for RFLPs. nP33 is a Hindlll RFLP detected by the "right" end of Jc8. The "right" end of Jc8 was made by digesting Jc8 with EcoRI and self-ligating. nP34 is a Hindlll RFLP detected by the "left" end of Jc8. The "left" end of Jc8 was made by digesting Jc8 by Sail and self ligating. nP36 and nP37 are both Hindlll RFLPs detected by T10H5 and B0564, respectively. Germ line transformation The procedure used for microinjection basically follows that of A. Fire (Fire, A., EMBO J. 5:2673-2680 (1986)). Cosmid DNA was twice CsCl gradient purified. Miniprep DNA was used when deleted cosmids were injected and was prepared from 1.5 ml overnight bacteria culture in superbroth. Superbroth was prepared by combining 12 g Bacto tryptone, 24 g yeast extract, 8 ml 50% glycerol and 900 ml H 2 O. The mixture was autoclaved and then 100 ml of 0.17 M KH 2 PO 4 and 0.72 M K 2 HPO 4 were added. The bacterial culture was extracted by the alkaline lysis method as described in Maniatis et al. (Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press (1983)). DNA was treated with RNase A (37°, 30 min) and then with protease K (55°, 30 min). The preparation was phenol- and then chloroform-extracted, precipitated twice (first in 0.3 M Na acetate and second in 0.1 M K acetate, pH 7.2), and resuspended in 5 1 injection buffer as described by A. Fire (Fire, A. , EMBO J. 5:2673-2680 (1986)). The DNA concentration for injection was in the range of 100 μg to 1 mg per ml. All transformation experiments used the ced-l(el735); unc-31(e928) ced-3(n717) strain, unc-31 was used as a marker for co-transformation (Kim et al , Genes & Dev. 4:357-371 (1990)). ced-1 was present to facilitate scoring of the ced-3 phenotype. The mutations in ced-1 block the engulfment process of cell death, which makes the coφses of the dead cells persist much longer than that in the wild-type (Hedgecock et al, Science 220: 1277-1280 (1983)). ced-3 phenotype was scored as the number of dead cells present in the head of young LI animals. The cosmid C10D8 or the plasmid subclones of C10D8 were mixed with C14G10 (wnc-57(+j-containing) at a ratio of 2: 1 or 3: 1 to increase the chances that an Unc-31(+) transformant would contain the cosmid or plasmid being tested. Usually, 20-30 animals were injected in one experiment. Non-Unc Fl progeny of injected animals were isolated three to four days later. About 1/2 to 1/3 of the non-Unc progeny transmitted the non-Unc phenotype to F2 and established a line of transfomants. The young Ll progeny of such non-Unc transformants were checked for the number of dead cells present in the head using Nomarski optics. Determination of ced-3 transcript initiation site. Two primers, Pexl: (5'GTTGCACTGCTTTCACGATCTCCCGTCTCT3') and Pex2: (5'TCATCGACTTTTAGATGACTAGAGAACATC3'), were used for primer extension. The primers for RT-PCR are: SLl (5'GTTTAATTACCCAAGTTTGAG3') and log-5 (5'CCGGTGACATTGGACACTC3'). The products are reamplified using the primers SLl and oligolO (5ΑCTATTCAACACTTG3'). A product of the expected length was cloned into the PCRIOOO vector (invitrogen) and sequenced. Determination and analysis of DNA sequence For DNA sequencing, serial deletions were made according to a procedure developed by Henikoff (Heinkoff, S., Gene 28:351-359 (1984)). DNA sequences were determined using Sequenase and protocols obtained from US Biochemicals with minor modifications. The ced-3 amino acid sequence was compared with amino acid sequences in the GenBank, PIR and SWISS-PROT databases at the National Center for Biotechnology Information (NCBI) using the blast network service. Cloning of ced-3 genes from other nematode species C. briggsae and C. vulgaris ced-3 genes were isolated from corresponding phage genomic libraries using the ced-3 cDNA subclone pJl 18 insert as a probe under low stringency conditions (5xSSPE, 20% Formamide, 0.02% Ficoll, 0.02% BSA, 0.02% polyvinylpyrrolidone, 1 % SDS) at 40°C overnight and washed in lxSSPE and 0.5 % SDS twice at room temperature and twice at 42 °C for 20 min each time. Results ced-3 is not essential for viability All previously described ced-3 alleles were isolated in screens designed to detect viable mutants in which programmed cell death did not occur (Ellis et al, Cell 44:817-829 (1986)). Such screens might systematically have missed classes of ced-3 mutations that result in inviability. Since animals with the genotype of ce-7-5/deficiency are viable (Ellis et al, Cell 44:817-829 (1986)), a noncomplementation-screening scheme was designed that would allow the isolation of recessive lethal alleles of ced-3. Four new ced-3 alleles (nll63, nll64, nll65, and nl286) were obtained which were viable as homozygotes. These new alleles were isolated at a frequency of about 1 in 2500 mutagenized haploid genomes, approximately the frequency expected for the generation of null mutations in an average C. elegans gene (Brenner, S., Genetics 77:71-94 (1974); Meneely et al, Genetics 92:99-105 (1990); Greenwald et al, Genetics 96: 147-160 (1980)). These results suggest that animals that lack ced-3 gene activity are viable. In support of this hypothesis, molecular analysis has revealed that three ced-3 mutations are nonsense mutations that prematurely terminate ced-3 protein translation and one alters a highly conserved splice acceptor site (see below). These mutations would be expected to eliminate ced-3 activity completely. Based upon these considerations, it was concluded that ced-3 gene activity is not essential for viability. ced-3 is contained within a 7.5 kb genomic fragment The ced-3 gene was cloned using the approach of Ruvkun et al. (Molecular Genetics of the Caenorhabditis Elgens Heterochronic Gene tin- 14 727:501-516 (1988)). Briefly (for further details, see Experimental Procedures), the C. elegans Bristol strain N2 contains 30 dispersed copies of the transposable element Tel, whereas the Bergerac strain contains more than 400 copies (Emmons et al, Cell 32:55-65 (1983); Finney, M., Ph.D. Thesis "The Genetics and Molecular Biology of unc-86, a Caenorhabditis elgens Cell Lineage Gene," Cambridge, MA (1987)). By crossing Bristol and Bergerac strains, a series of recombinant inbred strains were generated in which chromosomal material was mostly derived from the Bristol strain with varying amounts of Bergerac-specific chromosome IV-derived material in the region of the ced-3 gene. By probing DNA from these strains with plasmid pCe2001 which contains Tel (Emmons et al , Cell 32:55-65 (1983,) a 5.1 kb EcoRI Tcl-containing restriction fragment specific to the Bristol strain (restriction fragment length polymoφhism nP35) and closely linked to ced-3 was identified. Cosmids that contained this 5.1 kb restriction fragment were identified and it was found that these cosmids overlapped an existing cosmid contig that had been defined as part of the C. elegans genome project (Coulson et al, Proc. Natl. Acad. Sci. 85:7821-7825 (1986). Four other Bristol-Bergerac restriction fragment length polymoφhisms were defined by cosmids in this contig (nP33, np34, nP36, nP37). By mapping these restriction fragment length polymoφhisms with respect to the genes unc-30, ced-3 and unc-26, the physical contig was oriented with respect to the genetic map and the region containing the ced-3 gene was narrowed to an interval spanned by three cosmids (Fig. 1). By mapping these RFLPs between Bristol and Bergerac strains with respect to the genes unc-30, ced-3 and unc-26, the physical contig was oriented with respect to the genetic map. On Southern blot, three of three + Berg unc-26 recombinants showed the Bristol nP33 pattern while two of two ced-3 + Berg recombinants showed the Bergerac pattern (data not shown). Thus, nP33 maps very close or to the right of unc-26. For nP34, two of two ced-3 + Berg recombinants and two of three + Berg unc-26 recombinants showed the Bergerac pattern; one of the three + Berg unc-26 recombinant showed the Bristol pattern (data not shown). The genetic distance between ced-3 and unc-26 is about 0.2 mu. Thus, nP34 maps between ced-3 and unc-26, about 0.1 mu to the right of ced-3. Similar experiments mapped nP35, the 5.1 kb Bristol specific Tel element, to about 0.1 mu to the right of ced-3 (data not shown). In order to map n36 and n37, Bristol unc-30 ced-31 -V + males were crossed with Bergerac hermaphrodites. From the progeny of heterozygotes of genotype unc-30 ced-3 (Bristol)/ + + (Bergerac), Unc-30 non-ce7-5 and non- Unc-30 ced-3 animals were picked and DNA was prepared from these strains. nP36 maps very close or to the right of unc-30 since two of two unc-30 + Berg recombinants showed Bristol pattern and two of two + Berg ced-3 recombinants showed the Bergerac pattern (data not shown). Similarly, nP37 maps very close or to the right of unc-30 since four of the four + Berg ced-3 showed Bergerac pattern and six of six unc-30 + Berg recombinants showed the Bristol pattern (data not shown). These experiments narrowed the region containing the ced-3 gene to an interval spanned by the three cosmids (Fig. la). Cosmids that were candidates for containing the ced-3 gene were microinjected (Fire, A., EMBO J. 5:2673-2680 (1986)) into ced-3 mutant animals to test for rescue of the mutant phenotype. Specifically, cosmid C14G10, which contains the wild-type unc-31 gene and a candidate cosmid were coinjected into ced-1 (el 375); unc-31(e928) ced-3(n717) hermaphrodites. Non-unc progeny were isolated and observed to see if the non-Unc phenotype was transmitted to the next generation, thus establishing a line of transformed animals. Young LI progeny of such transformant lines were examined for the presence of cell deaths using Nomarski optics to see whether the ced-3 phenotype was complemented (see Experimental Procedures). Cosmid C14G10 alone does not confer ced-3 activity when injected into a ced-3 mutant. unc-31 was used as a marker for co-transformation (Kim et al , Genes & Devel. 4:357-371 (1990)). ced-1 was present to facilitate scoring of the ced-3 phenotype. Mutations in ced-1 block the engulfment process of programmed cell death, causing the coφses of dead cells to persist much longer than in the wild-type (Hedgecock et al, Science 220: 1277-1280 (1983)). Thus, the presence of a coφse indicates a cell that has undergone programmed cell death. The ced-3 phenotype was scored as the number of coφses present in the head of young LI animals. As indicated in Fig. 1, of the three cosmids injected (C43C9, W07H6 and C48D1), only C48D1 rescued the ced-3 mutant phenotype. Both non-Unc transformed lines obtained, nisi and nEx2, were rescued. Specifically, LI ced-1 animals contain an average of 23 cell coφses in the head, and LI ced-1; ced3 animals contain an average of 0.3 cell coφses in the head (Ellis et al, Cell 44:817-829 (1986)). By contrast, ced-1; unc-31 ced-3; nisi; and ced-1; unc-31 ced-3; nEx2 animals contained an average of 16.4 and 14.5 cell coφses in the head, respectively. From these results, it was concluded that C48D1 contains the ced-3 gene. To locate ced-3 more precisely within the cosmid C48D1, this cosmid was subcloned and the subclones tested for their ability to rescue the ced-3 mutant phenotype (Fig. IA). From these experiments, ced-3 was localized to a DNA fragment of 7.5 kb (pJ7.5). A 2.8kb ced-3 transcript is expressed primarily during embryogenesis and independently of ced-4 function The 7.6 kb pJ107 subclone of C48D1 (Fig. IA) was used as a probe in a northern blot of polyA+ RNA derived from the wild-type C. elegans strain N2. This probe hybridized to a 2.8 kb transcript. Although this transcript is present in 11 different EMS-induced ced-3 mutant strains, subsequent analysis has shown that all 11 tested ced-3 mutant alleles contain mutations in the genomic DNA that encodes this mRNA (see below), thus establishing this RNA as a ced-3 transcript. The developmental expression pattern of ced-3 was determined by hybridizing a northern blot of RNA from animals at different stages of development with the ced-3 cDNA subclone pJl 18 (see below). The ced-3 transcript was found to be most abundant during embryogenesis, when most programmed cell deaths occur, but was also detected during the LI through L4 larval stages. It is present in relatively high levels in young adults. Since ced-3 and ced-4 are both required for programmed cell death in C. elegans, and since both are highly expressed during embryonic development (Yuan et al, Dev. 776:309-320 (1992), the possibility existed that one of the genes might regulate the mRNA level of the other. Previous studies have revealed that ced-3 does not regulate ced-4 mRNA levels (Yuan et al, Dev. 776:309-320 (1992)). To determine if ced-4 regulates ced-3 mRNA levels, a northern blot of RNA prepared from ced-4 mutant embryos was probed with the ced-3 cDNA subclone pJ118. It was found that the amount and size of the ced-3 transcript was normal in the ced-4 mutants nll62, n!416, nl894 and n!920. Thus, ced-4 does not appear to affect the steady-state levels of ced-3 mRNA. ced-3 cDNA and Genomic Sequences To isolate ced-3 cDNA clones, ced-3 genomic DNA pJ40 (Fig. IA) was used as a probe to screen a cDNA library of the C. elegans wild-type strain N2 (Kim et al, Genes ά Dev. 4:357-371 (1990)). The 2.5 kb cDNA clone pJ87 was isolated in this way. On northern blots, pJ87 hybridized to a 2.8 kb transcript and on Southern blots, it hybridized only to bands to which pJ40 hybridizes (data not shown). Thus, pJ87 represents an mRNA transcribed entirely from pJ40 which can rescue the ced-3 mutant phenotype when microinjected into ced-3 mutant animals. To confirm that pJ87 contains the ced-3 cDNA, a frameshift mutation in the Sail site of pJ40 was made corresponding to the Sail site in the pJ87 cDNA. Constructs containing the frameshift mutation failed to rescue the ced-3 phenotype when microinjected into ced-3 mutant animals (6 transformant lines; data not shown), suggesting that ced-3 activity had been eliminated by mutagenizing a region of genomic DNA that corresponds to the pJ87 cDNA. The DNA sequence of pJ87 is shown in Figure 2C. pJ87 contains an insert of 2482 bp with an open reading frame of 503 amino acids. It has 953 bp of 3' untranslated sequence, not all of which is essential for ced-3 expression; genomic constructs that do not contain 380 bp of the 3 '-most region (pJ107 and its derivatives, see Fig. la) were capable of rescuing ced-3 mutant phenotype. The cDNA ends with a poly-A sequence, suggesting that the complete 3' end of the ced-3 transcript is present. To confirm the DNA sequence obtained from the ced-3 cDNA and to study the structure of the ced-3 gene, the genomic sequence of the ced-3 gene from the plasmid pJ107 was determined. The insert in pJ107 is 7656 bp in length (Fig. 2). To determine the location and nature of the 5' end of the ced-3 transcript, a combination of primer extension and amplification using the polymerase chain reaction (PCR) was used. Two primers, Pexl and Pex2, were used for primer extension. The Pexl reaction yielded two major bands, whereas the Pex2 reaction gave one band. The Pex2 band corresponds in size to the smaller band from the Pexl reaction, and agrees in length with a possible transcript that is trans-spliced to a C. elegans splice leader (Bektesh et al., Genes and Dev. 2: 1277-1283 (1988)) at a consensus splice acceptor at position 2166 of the genomic sequence. The nature of the larger Pexl band is unclear. To confirm these observations, wild-type total RNA was reverse- transcribed and then amplified using the primers SLl and log-5 followed by reamplification using the primers SLl and oligolO. A product of the expected length was cloned into the PCRIOOO vector (invitrogen) and sequenced. The sequence obtained confirmed the presence of a ced-3 message trans-spliced to SLl at position 2166 of the genomic sequence. These experiments suggest that a ced-3 transcript is trans-spliced to the C. elegans splice leader SLl (Bektesh et al, Genes and Dev. 2: 1277-1283 (1988)) at a consensus splice acceptor at position 2166 of the genomic sequence. Based upon these observations, it is concluded that the start codon of ced-3 protein is the methionine encoded at position 2232 of the genomic sequence and that the ced-3 protein is 503 amino acids in length. The predicted ced-3 protein is hydrophilic (256/503 residues are charged or polar) and does not contain any obvious potential trans-membrane domains. One region of the ced-3 protein is rich in serines: from amino acid 107 to amino acid 205, 32 of 99 amino acids are serine residues. The sequences of 12 EMS-induced ced-3 mutations (Table 1) were determined. Eight are missense mutations, three are nonsense mutations, and one alters a conserved G at the splice acceptor site of intron 6. Interestingly, nine of these 12 mutations alter residues within the last 100 amino acids of the protein, and none occurs within the serine-rich region. Table 1. Sites of mutations in the ced-3 gene. Allele Mutation Nucleotide Codon Consequence n717 G to A 6297 Altered splicing n718 G to A 2487 65 G to R nl040 C to T 2310 27 L to F nll29 & nl64 C to T 6434 449 A to V nll63 C to T 7020 486 S to F nll65 C to T 5940 403 Nonsense nl286 G to A 6371 428 Nonsense n!949 C to T 6222 412 Nonsense n2426 G to A 6535 483 E to K n2430 C to T 6485 466 A to V n2433 G to A 5757 360 G to S Nucleotide and codon positions correspond to the numbering in Fig. 2. To identify functionally important regions of the ced-3 protein, the genomic sequences of the ced-3 genes from the related nematode species C. briggsae and C. vulgaris were cloned and sequenced. Sequence comparison of the three ced-3 genes showed that the relatively non-serine-rich regions of the proteins are more conserved than are serine-rich regions (Fig. 3A). All 12 EMS-induced ced-3 mutations altered residues that are conserved among the three species. These results suggest that the non-serine-rich region is important for ced-3 function and that the serine rich region is either unimportant or that residues within it are functionally redundant. ced-3 protein is similar to the mammalian ICE and Nedd-2 proteins A search of the GenBank, PIR and SWISS-PROT databases revealed that the non-serine-rich regions of the ced-3 protein are similar to human and murine interleukin-1/3 (IL-1/3) convertases (ICE) (Fig. 3A). 7CE is a cysteine protease that cleaves the inactive 31 KD precursor of IL-1/3 between Asp 116 and Ala 117 releasing a carboxy-terminal 153 amino-acid peptide known as mature IL-lβ (Kostura et al, Proc. Natl Acad. Sci., USA 86:5227-5231 (1989); Black et al, FEBS Lett. 247:386-390 (1989)). The most highly conserved region among the proteins shown in Figure 3A consists of amino acids 246- 360 of the ced-3 protein and amino acids 166-287 of the human 7CE protein: 49 residues are identical (43 % identity). The active site cysteine of human 7CEis located at cysteine 285 (Thornberry et al, Nature 556:768-774 (1992)). The five-amino-acid peptide (QACRG) around this active cysteine is the longest conserved peptide among the murine and human 7CE proteins and ced-3 proteins from nematodes. Human 7CE is composed of two subunits (p20 and plO) that appear to be proteolytically cleaved from a single proenzyme by the mature enzyme (Thornberry et al, Nature 556:768-774 (1992)). Two cleavage sites in the proenzyme, Asp-Ser at positions 103 and 297 oi ICE, are conserved in ced-3 (position 131 and 371, respectively). The C-terminal portion of the ced-3 protein and the p 10 subunit oilCE are similar to the protein product of the murine nedd-2 gene, which is highly expressed during embryonic brain development and is down-regulated in adult brain (Kumar et al, Biochem and Biophy. Res. Comm. 785: 1155-1161 (1992)). The ced-3 and nedd-2 proteins, and the 7CE and nedd-2 proteins are 27% identical (Fig. 3A). The nedd-2 protein does not contain the QACRG peptide at the active site of ICE (Fig. 3A). Seven of eight point mutations that were analyzed («778, «7040, «7729, «7764, n2430, n2426 ά n2433) result in alterations of amino acids that are conserved or semi-conserved among the three nematode ced-3 proteins, 7CE and the nedd-2 protein. In particular, the mutation, n2433, introduces a Gly to Ser change near the putative active cysteine (Fig. 2, Table 1). Discussion The genes ced-3 and ced-4 are the only genes known to be required for programmed cell death to occur in C. elegans (Ellis et al, Cell 44:817-829 (1986)). Genetic and molecular studies have revealed that the ced-3 gene shares a number of features with ced-4. Like ced-4 (see Yuan et al, Dev. 776:309-320 (1992)), ced-3 is not required for viability. It appears to encode a single mRNA which is expressed mostly in the embryo, the stage during which 113 of the 131 programmed cell death occur. Furthermore, just as ced- 3 gene function is not required for ced-4 gene expression (Yuan et al, Dev. 776:309-320 (1992)), ced-4 gene function is not required for ced-3 gene expression. Thus, these two genes do not appear to control the onset of programmed cell death by acting sequentially in a transcriptional regulatory cascade. Unlike ced-4 (Yuan et al , Dev. Biol. 758:33-41 (1992)), ced-3 is expressed at a substantial level in young adults, this observation suggests that ced-3 expression might not be limited to cells undergoing programmed cell death. The ced-4 protein is novel in sequence, and the only hint concerning its function is that two regions of the protein show some similarity to the EF- hand motif, which binds calcium (Yuan et al, Dev. 776:309-320 (1992)). For this reason it has been suggested that the ced-4 protein and hence programmed cell death in C. elegans might be regulated by calcium. However, no direct evidence for this hypothesis has yet been obtained. The ced-3 protein similarly contains a region that offers a clue about possible biochemical function: a region of 99 amino acids contains 32 serines. Since serines are common phosphorylation sites (Edelman et al, Ann. Rev. Biochem. 56:567- 613 (1987)), it is possible that the ced-3 protein and hence programmed cell death in C. elegans is regulated by phosphorylation. Phosphorylation has previously been suggested to function in cell death. McConkey et al. (McConkey et al, J. Immunol. 745: 1227-1230 (1990)) have shown that several agents that can elevate cytosolic cAMP level induce thymocyte death, suggesting that protein kinase A may mediate cell death by phosphorylating certain proteins. Although the precise sequence of the serine-rich region varies among the three Caenorhabditis species studied, the relatively high number of serines is conserved in C. elegans, C. briggsae and C. vulgaris. None of the mutations in ced-3 affect the serine-rich region. These observations are consistent with the hypothesis that the presence of serines is more important than the precise amino acid sequence within this region. Much more striking than the presence of the serine-rich region in the ced-3 protein is the similarity between the non-serine-rich regions of cecf-5 and the human and murine interleukin-1 / 3 converting enzyme (ICE). Human 7CE is a substrate-specific protease that cleaves 31 KD prointerleukin-1/3 at Asp 116 - Ala 117 to produce the mature 17.5 kD interleukin-13 (IL-lβ). IL-1/3 is a cytokine involved in mediating a wide range of biological responses including inflammation, septic shock, wound healing, hematopoiesis and growth of certain leukemias (Dinarello, C.A., Blood 77:1627-1652 (1991); diGiovine et al, Today 77: 13 (1990)). A specific inhibitor oi ICE, the crmA gene product of Cowpox virus, prevents the proteolytic activation of interleukin-1/3 (Ray et al, Cell 69:597-604 (1992)) and inhibits host inflammatory response (Ray et al, Cell 69:597-604 (1992)). Cowpox virus carrying a deleted crmA gene is unable to suppress the inflammatory response of chick embryos, resulting in a reduction in the number of virus-infected cells and less damage to the host (Palumbo et al, Virology 777:262-273 (1989)). This observation indicates the importance oi ICE in bringing about the inflammatory response. The carboxy half of the ced-3 protein is the region most similar to 7CE. A stretch of 115 residues (amino acids 246-360 of ced-3) is 43% identical between the ced-3 and 7CE proteins. This region contains a conserved pentapeptide QACRG (positions 361-365 of the ced-3 protein), which surrounds the active cysteine oilCE. Specific modification of this cysteine in human 7CE results in complete loss of activity (Thronberry et al, Nature 556:768-774 (1992)). The ced-3 mutation n2433 alters the conserved glycine in this pentapeptide and eliminates ced-3 function, suggesting that this glycine is important for ced-3 activity and might be an integral part of the active site of 7CE. Interestingly, the mutations n718 (position 67 of ced-3) and t.7040 (position 27 of ced-3) eliminate ced-3 function in vivo yet they contain alterations in conserved residues which are outside of mature P20 subunit of 7CE (Thronberry et al, Nature 556:768-774 (1992)). Perhaps these residues have a non-catalytic role in both ced-3 and 7CE function, e.g. they may maintain a proper conformation for proteolytic activation. The ICE precursor (p45) is proteolytically cleaved at 4 sites of 7CE (Aspl03, Aspl l9, Asp297 and Asp316) to generate p24, p20, and plO (Thronberry et al, Nature 556:768-774 (1992)). At least two of the cleavage sites are conserved in ced-3 indicating that the ced-3 product might be processed as well. The similarity between the ced-3 and 7CE proteins strongly suggests that ced-3 might function as a cysteine protease in controlling programmed cell death by proteolytically activating or inactivating a substrate protein. One potential substrate for ced-3 might be the product of the ced-4 gene which contains 6 Asp residues that might be the target of ced-3 protein (Asp25, Aspl51, Aspl85, Aspl92, Asp459 and Asp541). Alternatively, the ced-3 protein might directly cause cell death by proteolytically cleaving certain proteins or subcellular structures that are crucial for cell viability. ced-3 and 7CE are part of a novel protein family. Thornberry et al. suggested that the sequence GDSPG at position 287 of 7CE resembles a GX(S/C)XG motif found in serine and cysteine protease active sites (Nature 556:768-774 (1992)). However, in the three nematode ced-3 proteins examined, only the first glycine of this sequence is conserved and in mouse 7CE the S/C is missing. This suggests that the ced-3/lCE family shares little sequence similarity with known protease families. The similarity between ced-3 and 7CE suggests, not only that ced-3 functions as a cysteine protease, but also that 7CE functions in programmed cell death in vertebrates. Consistent with this hypothesis, it has been observed that after murine peritoneal macrophages are stimulated with lipopolysaccharide (LPS) and induced to undergo programmed cell death by exposure to extracellular ATP, mature active IL-lβ is released into the culture supernatant. In contrast, when cells are injured by scraping, IL-1 / 3 is released exclusively as the inactive proform (Hogoquist et al, Proc. Natl Acad. USA 88:8485-8489 (1991)). These results suggest that 7CE is activated upon induction of programmed cell death. ICE transcript has been detected in cells that do not make I-L-l/3 (Cerretti et al, Science 256:97-100 (1992)), suggesting that other 7CE substrates exist. This suggests that ICE could mediate programmed cell death by cleaving a substrate other than IL-1/3. The carboxy-terminal portions of both the ced-3 protein and the plO subunit of ICE are similar to the protein encoded by the murine nedd-2 gene, which is expressed preferentially during early embryonic brain development (Kumar et al, Biochem and Biophy. Res. Comm. 785: 1155-1161 (1992)). Since the nedd-2 protein lacks the QACRG active domain, it might function to regulate an 7CE or 7CE-like p20 subunits. Interestingly, four ced-3 mutations alter residues conserved between the nedd-2 and ced-3 proteins and nedd-2 gene expression is high during embryonic brain development, when much programmed cell death occurs. These observations suggest that nedd-2 might function in programmed cell death. The C. elegans gene ced-9 protects cells from undergoing programmed cell death by directly or indirectly antagonizing the activities of ced-3 and ced- 4 (Hengartner et al, Nature 556:494-499 (1992)). The vertebrate gene bcl-2 acts in a way functionally similar to ced-9. Overexpression of bcl-2 protects or delays the onset of apoptotic cell death in a variety of vertebrate cell types as well as in C. elegans (Vaux et al, Science 258: 1955-1957 (1992); Nunez et al, J. Immun. 744:3602-3610 (1990); Vaux et al, Science 258: 1955-1957 (1992); Sentman et al, Cell 67:879-888 (1992); Strasser et al, Cell 67:889- 899 (1991)). Thus, if 7CE or another ced-3/ICE family member is involved in vertebrate programmed cell death, it is possible that bcl-2 could act by modulating its activity. The fact that bcl-2 is a dominant oncogene (overexpression of bcl-2 as a result of chromosomal translocation occurs in 85 % of follicular and 20% of diffuse B cell lymphomas, Fukuhara et al , Cancer Res. 59:3119 (1979); Levine et al, Blood 66: 1414 (1985); Yunis et al , N. Engl J. Med. 576:79-84 (1987)) suggests that 7CE and other ced-3/ICE family members might be recessive oncogenes. The elimination of such cell death genes would prevent normal cell death and promote malignancy, just as does overexpression of bcl-2. Example 2 The mouse homolog of human 7CE from a mouse thymus cDNA library (Stratagene) was cloned by low stringency hybridization using human 7CE as a probe. This clone, named "mICE", is identical to the clone isolated by Net et al. (J. Immun. 749:3245-3259 (1992)) except that base pair 166 is an A and, as a result, Asn is encoded rather than Asp. This may be a DNA polymoφhism since the isolated clone was from a thymus cDNA library (Stratagene) of mouse B6/CBAF1J (C57Black x CBA) strain while Nett's clone was from a WΕH13 cell cDNA library (Stratagene). Subsequent experiments have shown that this DNA polymoφhism is in a region which is not essential for 7CE function (see below). Thus, the presence of Asn rather than Asp should have no effect on the results obtained. In order to circumvent the difficulty of establishing a permanent cell line that expresses 7CE in high levels, a transient expression system was developed to determine if overexpression of m7CE kills cells. mICE cDNA was fused with the E. coli lac-Z gene and the product so produced was placed under the control of chicken j8-actin promoter (Fig. 4). The active 7CE protein is known to have two subunits, P20 and PlO, which are processed from a precursor peptide (Thornberry et al. , Nature 556:768-774 (1992)). To test the function of the subunits, two additional fusion genes were made, P20/P10-lacZ and PlO-lacZ. The constructs shown in Fig. 4 were transfected into rat 1 cells by calcium phosphate precipitation. 24 hours after transfection, cells were fixed and X-gal was added to begin the color reaction. It was found that, after 3 hours of color development, most blue cells transfected with intact mICE-lacZ or P20/P10-lacZ were round, whereas most blue cells transfected with P70- lacZ or the control lac-Z construct were normal, flat cells (Table 2). Similar results were obtained with another cell line, NG108-15 neuronal cells. Healthy living rat cells are flat and well-attached to plates whereas dying cells are round and often float into the medium. Table 2. Overexpression of mICE causes rat-1 cells to undergo programmed cell death. The constructs shown in Fig. 4 are transiently transfected into rat-1 cells, rat-1 cells expressing bcl-2 (τst-l/bcl-2) or rat-1 cells expressing crmA (τat-l/crmA). 24 hrs after transfection, cells are fixed and stained with X-gal for 3 hrs. The data shown are the percentage of round blue cells among total number of blue cells. The data are collected from at least three different experiments. Construct rat-1 rat-l/bcl-2 rat-1/crmA pActbGal' 1.44 ± 0.18 2.22 ± 0.53 2.89 ± 0.79 pøActMIOZ 80.81 ± 2.33 9.91 ± 2.08 18.83 ± 2.86 piSActMllZ 93.33 + 2.68 13.83 ± 4.23 24.48 ± 2.78 p/?actM19Z 2.18 ± 0.54 p/3ActM12Z 2.44 ± 0.98 3.33 ± 1.45 2.55 ± 0.32 pSactl7Z 2.70 ± 1.07 pJ485 1.32 ± 0.78 p/3Actced38Z 46.73 ± 4.65 35.28 ± 1.36 34.40 ± 2.38 p/5Αctced37Z 3.67 ± 1.39 - - Methods: a: Construction of bcl-2 expressing vector (pJ415): pJ415 was constructed by first inserting 5', the 400bp BglH/BαmUl crmA fragment into the Bαmϋl site of the pBabe/puro vector and then inserting the remaining lkb BαmHl crmA fragment into the 3'BαmHl site in the sense direction, b: Construction of the bcl-2 expressing vector (pJ436): pJ436 was constructed by inserting an EcoRI/ Sail bcl-2 fragment into the EcoRl/Sall sites of the pBabe/puro vector, c: Establishing Rat-1 cell lines that overexpress crmA and bcl-2: pJ415 and pJ436 were electroporated into CRE retroviral packaging cells (Danos et al, Proc. Natl. Acad. Sci. U.S.A. 85:6460-6464 (1988)) using a BioRad electroporating apparatus. Supernatant either from overnight transiently transfected CRE cells or from stable lines of CRE cells expressing either crmA or bcl-2 were used to infect Rat-1 cells overnight in the presence of 8 μg/ml of polybrene. Resistant cells were selected using 30 ug/ml puromycin for about 10 days. Resistant colonies were cloned and checked for expression using both Northern and Western blots. Bcl-2 antibodies were from S.J. Korsmeyer and from DAKO. crmA antisera was made by immunizing rabbits with anm E. cotϊ-expressed crmA fusion protein (pJ434). pJ434 was made by inserting an EcoRl/Sall fragment of crmA cDNA into EcoRI/ Sail sites of pΕT21a (Novagen) and fusion protein was expressed in the E. coli BL21 (DE3) strain. Multiple lines that express either bcl-2 or crmA were checked for suppression of mICE induced cell death and all showed similar results. When cells were stained with rhodamine-coupled anti-/3 galactosidase antibody and Hoechst dye, it was found that galactosidase-positive round cells had condensed and fragmented nuclei. Such nuclei are indicative of programmed cell death. When observed in an electron microscope, the X-gal reaction product was electron dense, allowing ICE-lacZ expressing cells to be distinguished from other cells (Snyder et al, Cell 68:33-51 (1992)). The chimeric gene expressing cells showed condensed chromatin and membrane blebbing. These are characteristics of cells undergoing programmed cell death (Wyllie, A.H., in Cell Death in Biology and Pathology, 9-34 (1981); Oberhammer et al, Proc. Natl. Acad. Sci. U.S.A. 89:5408-5412 (1992); Jacobson et al , Nature 567:365-369 (1993)). Thus, the results indicate that overexpression oimlCE induces programmed cell death and induction depends on both P20 and PlO subunits. When color development in rat-1 cells transfected with mICE-lacZ or P20/P10-lacZ is allowed to proceed for 24 hours, a greater number of flat cells turn blue. This result indicates that a lower level oi ICE activity can be tolerated by cells. If mICE is a vertebrate homolog of ced-3, then ced-3 might also be expected to cause cell death in vertebrates. This hypothesis was tested by making a ced-3-lacZ fusion construct and examining its ability to cause cell death using the assay as described above. As expected, the expression oi ced-3 caused the death of rat cells (Table 2). If m7CE functions in a similar way to ced-3, another prediction is that mutations eliminating ced-3 activity in C. elegans should also eliminate its activity in vertebrates. This hypothesis was tested by mutating the Gly residue in the pentapeptide active domain of 7CE, QACRG, to Ser. It was found that this mutation eliminated the ability of both mICE and ced-3 to cause rat cell death (Table 2). The cowpox gene crmA encodes a 38 kD protein that can specifically inhibit ICE activity (Ray et al, Cell 69:597-604 (1992)). To demonstrate that cell death caused by overexpression of mICE is due to the enzymatic activity of ICE protein, rat-1 cells were infected with a pBabe retroviral construct (Morgenstern et al , Nucl. Acids Res. 78:3587-3596 (1990)) expressing crmA and cell lines were identified which produce a high level of crmA protein. When the mlCE-lacZ construct was transfected into these cell lines, it was found that a large percentage of blue cells had a healthy, flat moφhology (Table 2). In addition, a point mutation that changes the Cys residue in the active site pentapeptide, QACRG to a Gly eliminates the ability of 7CE to cause cell death (construct pj8actM17Z, Figure 4, Table 2). This result indicates that the proteolytic activity of 7CE is essential to its ability to kill cells. In mammals, bcl-2 prevents certain cells from undergoing programmed cell death (Vaux et al, Nature 555:440-442 (1988); Nunez et al, J. Immun. 744:3602-3610 (1990); Strasser et al, Cell 67: 889-899 (1991); Sentman et al, Cell 67:879-888 (1991)). Expression oi bcl-2 in the nematode C. elegans has been shown to partially prevent programmed cell death. Thus, bcl-2 is functionally similar to the C. elegans ced-9 gene (Vaux et al, Science 258: 1955-1957 (1992); Hengartner et al, nature 556:494-499 (1992)). Rat-1 cells were infected with the pBabe retroviral construct expressing bcl-2. Transfection of the mICE-lacZ fusion construct into the cells lines overexpressing bcl-2 showed that a high percentage of blue cells were now healthy (Table 2). Thus, cell death induced by overexpression of mICE can be suppressed by bcl-2. This result indicates that cell death induced by overexpression of 7CE is probably caused by activation of a normal programmed cell death mechanism. The amino acid sequence of 7CE is similar to C. elegans ced-3, which functions in initiating programmed cell death during development. Thus, vertebrate animals may have a genetic pathway of programmed cell death similar to that of C. elegans (Fig. 5). Example 3 As described above, the genes in the lCEIced-3 family would be expected to function during the initiation of programmed cell death. In order to identify additional members of this gene family, cDNA encoding human interleukin-l/3 converting enzyme (ICE) was used to screen a mouse thymus cDNA library (Stratagene) under conditions of low stringency. Using this procedure, a new gene was identified and named "mICE2" (see Figure 6 for the cDNA sequence and deduced amino acid sequence of mlCEl). Figures 7 and 7A shows that the protein encoded by mlCE2 contains significant homology to both human and murine interleukin-lβ converting enzyme (ICE), as well as to the C. elegans cell death gene, ced-3. The sequence homology indicates that mICE2, like mICE, is a vertebrate cell death gene. Northern blot analyses showed that, unlike mICE, which is broadly expressed during embryonic development, the expression of mlCE2 is restricted to the thymus and placenta, areas where cell death frequently occurs. In addition, it was found that the expression of mlCE2 in the thymus can be induced by dexmethosome, an agent which causes thymus regression. It is concluded that mlCE2 is a thymus/placenta specific vertebrate cell death gene. Example 4 Extensive cell death occurs in the developing nervous system (Oppenheim, R. W., Ann. Rev. Neurosci. 745:453-501 (1991)). Many neurons die during the period of synapse formation. During this critical period, the survival of neurons depends on the availability of neural trophic factors. The survival of isolated primary neurons in vitro depends critically on the presence of such trophic factors (Davies, A. M., Development 700: 185- 208 (1987)). Removal of such factors induces neuronal cell death, usually within 48 hrs. The death of the sympathetic neurons and sensory neurons whose survival depends on one or more members of the nerve growth factor family (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3) can be rescued by microinjection of bcl-2 expression vector (Garcia, I., et al , Science 258:302-304 (1993); Allsopp et al, 1993). To examine if the genes in the Ice/ced-3 family may be responsible for neuronal cell death, the ability of crmA to inhibit the death of chicken dorsal root ganglionic neurons induced by NGF removal was examined. It was found that microinjection of an expression vector containing crmA inhibits the death of DRG neurons as effectively as that of a bcl-2 expression vector (Gagliardini, V., et al , Science 265:826-828 (1994)). This result demonstrated that the genes in the lcelced-3 family may play a key role in regulating neuronal cell death during development. Example 5 Results Cloning of Ich-1 The protein product of the C. elegans cell death gene, ced-3, is homologous to the product of the mouse gene, nedd-2, isolated by Kumar et al. as part of a group of genes that are down regulated during late mouse brain development (Kumar et al, Biochem. Biophys. Res. Commun. 785:1155- 1161 (1992); Yuan, J., et al , Cell 75:641-752 (1993)). The nedd-2 cDNA in the data bank has an open reading frame of 171 amino acids and has long 3' and 5' untranslated regions. This 171-amino acid nedd-2 protein does not contain the active domain, QACRG, of ICE and ced-3 proteins and is homologous only to the PlO subunit of mammalian interleukin-ljδ converting enzyme (ICE) and the C-terminal part of the ced-3 protein. While analyzing nedd-2 cDNA, the inventors discovered that it contains the sequence that can potentially encode a QACRG pentapeptide, but that the sequence is in another reading frame. The inventors considered the possibility that the nedd-2 cDNA isolated by Kumar et al. contains cloning artifacts and that another nedd-2 transcript could encode a protein homologous to both the P20 and PlO subunits of 7CE. A mouse nedd-2 probe was made by polymerase chain reaction (PCR). Using this mouse nedd-2 probe, three cDNA libraries were screened: a mouse embryonic day 11.5 cDNA library from CLONTΕCH (one million clones screened), a human fetal brain cDNA library from James Gusella's laboratory (10 million clones screened) and a human fetal brain cDNA library from Stratagene (one million clones screened). The longest positive cDNA clones were obtained from the Stratagene cDNA library. From the Stratagene library, two cDNA species (pBSH37 and pBSH30) were identified that encode two closely related proteins homologous to the mouse nedd-2 protein. The insert of pBSH37 (2.5 kb) encodes a protein that contains amino acid sequence similarities to both the P20 and PlO subunits of 7CE and entire ced-3 protein. The insert of pBSH30 (2.2 kb) contains a 61 bp additional sequence one basepair after the sequence encoding QACRG which causes an early termination of protein translation. The Northern blot analysis showed that expression patterns of this human gene are different from the expression of nedd-2 reported by Kumar et al. (see below); thus, the sequences were renamed Ich-1 L (pBSH37) (Figure 12A) and Ich-l s (pBSH30) (Figure 12B). Ich-l s cDNA differs from lch-1 L at two locations. The first difference is at the beginning of the coding region. The putative first methionine of lch-1 s is 15 amino acids downstream from the first methionine of Ich-1 L because the beginning 35 bp of lch-1 s is different from lch-1 L and includes a stop codon (Figure 12B). PCR analysis using primers specific to the first 35 bp of lch-1 and the 7c/z-7 s -specific intron (see below) and human placenta cDNA as template amplified a DNA fragment of predicted size, suggesting that the 35 bp 7c/z-7 s -specific sequence is not a cloning artifact and is present in the endogenous lch-1 s mRNA (data not shown). The second difference is after the active domain QACRG. lch-1 s begins to differ from lch-1 L one basepair after the coding region of the active site QACRG. The difference is caused by an insertion of 61 bp sequence, which results in a termination codon 21 amino acids downstream from the insertion. The last two identical basepairs of lch-1 s and lch-1 L are AG, the general eukaryotic splicing donor consensus sequence (Mount, 1982). Mouse genomic DNA of lch-l was cloned. Analysis of mouse genomic lch-1 DNA showed that the 61 bp is from an intron, whose sequence is identical between human and mouse lch-1. This difference between 7ctV7 s and lch-1 L is caused by alternative use of two different 5' splicing donor sequences. A schematic diagram of lch-1 1 and lch-1 s is shown in Fig. 13. As the result of an insertion of an intron between coding regions, the open reading frame of lch-1 s is broken into two: the first one encodes a 312 amino acid peptide homologous to the P20 subunit of 7CE only and the second encodes a 235 amino acid peptide homologous to a part of the P20 subunit and the PlO subunit of 7CE. The second is almost identical to the mouse nedd-2 protein (Figures 12 and 13). The data suggest that only the first reading frame is translated in cells (see below). lch-1 L protein contains similarities to both 7CE (27% identity and 52% similarity) and ced-3 (28% identity and 52% similarity) (Figure 14). Thus, the homology between lch-1 and ced-3, lch-1 and 7CE is about equal. lch-1 is expressed in many tissues and THP-1 cells which express interleukin-lβ converting enzyme To characterize the function of lch-1, the expression pattern oi lch-1 was examined. Northern blot analysis of human fetal heart, brain, lung, liver and kidney tissue using the insert of pBSH37 as a probe hybridizing to both 7cΛ-7 5 and lch-1 L transcripts, revealed that 4 kb lch-1 mRNA is expressed at low level in about same amount in all tissues examined. When the same Northern blot was analyzed using Ich-l s 61bp intron as a probe (which hybridizes to lch-1 s transcript only), it showed that Ich-l s was expressed in a larger amount in the embryonic heart and brain than in the lung, liver and kidney. This result suggests that in the embryonic lung, liver and kidney, lch-1 L is expressed in a larger amount than lch-1 s . In Northern blot analysis of adult RNA with pBSH37 probe, lch-1 is detected in all the tissues examined: its level is higher in placenta, lung, kidney, pancreas than in heart, brain, liver and skeletal muscle. To examine whether lch-1 and 7CE are expressed in the same cells, a Northern blot of THP-1 cells was analyzed. 7ce expression has been detected in these cells (Thornberry, N. A. , et al , Nature 556:768-774 (1992); Cerretti, D. P., et al, Science 256:97-100 (1992)). The inventors found t at lch-l can be detected in THP-1 cells. Thus, lch-1 and 7CE are both expressed in THP-1 cells. Using quantitative RT-PCR, we examined the expression of 7CE and lch-1 in the normal living T-cell hybridoma DO11.10 cells (Haskins, K., et al , Exp. Med. 757: 1149-1169 (1983)) as well as dying DO11.10 cells in serum-deprived condition. Similar to THP-1 cells, the expression of both ICE and 7c/z-7 can be detected in DO 11.10 cells. Interestingly, the expression levels of both Ich-1 L and 7CE appear to increase in dying DO 11.10 cells under serum-deprived condition. Overexpression of lch-1 L induces rat-1 fibroblast death To examine the function of lch-1 L , the same transient expression system used for 7CE (Miura, M., et al, Cell 75:653-660 (1993)) was used to determine if overexpression of lch-1 induces programmed cell death. The human Ich-1 L cDNA was fused with the Escherichia coli lacZ gene and the fused gene was placed under the control of the chicken β-actin promotor (p/3actH37Z). This fusion gene was transfected into Rat-1 cells by lipofectamine mediated gene transfer and the expression of the gene was examined using the X-gal reaction. Results showed that most of the blue (X-Gal-positive) Rat-1 cells transfected with p/3actH37Z were round. These results are similar to those obtained with cells transfected with mlce-lacZ fusion sequence (Table 1). In contrast, most blue cells transfected with vector alone were flat and healthy. Live Rat-1 cells are flat while dying Rat-1 cells are round and eventually detached from plates. This result suggests that the expression of lch-1 L induces Rat-1 cells to die. To examine whether the cell death induced by lch-1 has any cell type specificity and to compare its effect with that oilCE, mice- lacZ and lch-1 -lacZ fusion constructs were transfected to HeLa cells, NG108-15 cells, and COS cells. The cell killing effect was assayed as before (Table 1). The results showed that compared to controls, the cytotoxic effect of lch-1 and 7CE exhibit certain cell type specificities. Expression of either lch-1 or 7CE kill Rat-1 cells and HeLa cells effectively ( > 90% dead). NG108 cells are more resistant to lch-1 and 7CE expression than Rat-1 cells and HeLa cells (68-80% dead). Expression of either lch-1 or 7CE cannot kill COS cells (Table 1). To examine the nuclear moφhology of the cell death induced by lch-1 expression, the lch-1 L -lacZ Rat-1 cell transfectants were stained with a rhodamine-coupled anti-/3-galactosidase antibody and Hoechst dye. Results showed that /3-galactosidase-positive round cells have condensed and fragmented nuclei. This is one of the characteristics of cells undergoing apoptosis. Thus, the results suggest that overexpression of lch-1 L , like that of ICE, causes Rat-1 cells to undergo programmed cell death. To determine if cell death caused by overexpression of Ich-1 L is specific, three mutant lch-1 L fusion proteins were made: the first was a Ser → Cys 303 in the active site of lch-1, the second was a Thr → Ala 352 in the putative PlO subunit and the third with a Phe → Leu 212 in the putative P20 subunit (Fig. 14). The Ala 352 in PlO and Leu 212 in P20 are two amino acid residues of ced-3 that are conserved in lch-1 but not in ICE. The mutant lch-1 L -lacZ fusion constructs were transfected into Rat-1 cells and the expression was examined by X-gal reaction as before. The analysis revealed that the S303C and T352A mutations eliminated the activity of lch-1 completely (Table 1) while F212L mutation caused a reduction of cell killing activity of lch-1 L (Table 1). These results suggest that the ability of 7cΛ-7 to cause cell death depends upon its enzymatic activity and that only some characteristics of ced-3 are conserved in lch-1. The cell death induced by overexpression of 7ce can be inhibited by bcl-2 and crmA (Miura, M., et al , Cell 75:653-660 (1993)). To examine if the cell death induced by expression of lch-1 could also be inhibited by bcl-2 and crmA, Ich-l L -lacZ fusion construct was transfected into Rat-1 cells that overexpress either bcl-2 or crmA (Miura, M., et al , Cell 75: 653-660 ( 1993)) . Cell death was assayed as described for Table 1. The results showed that the cell death induced by overexpression oilch-l could be inhibited effectively by bcl-2 but only marginally by crmA. Expression of Ich-l s protects Rat-1 fibroblast death Since Ich-l s contains two open reading frames, it was important to determine which reading frame is functionally translated, lch-1 s was translated in the presence of 35 S-methionine using in vitro transcribed RNA in a reticulocyte lysate as described in Experimental Procedures. The translated products were run on an SDS-polyacrylamide gel with molecular weight standards. lch-1 s antisense RNA was used as a negative control. Results showed that only the first reading frame was translated. Second, E. coli lacZ gene was fused to the ends of first (p/3actH30Zl) and second (p/3actH30Z2) open reading frames. The constructs were separately transfected into Rat-1 cells and the cells were assayed for color using the X-gal reaction. Results showed that only when LacZ gene was fused to the end of the first open reading frame (but not the second open reading frame) could blue cells be detected. Thus, it is most likely that only the first open reading frame of lch-1 s homolog is used in vivo. To characterize the function of Ich-l s , the ability of pβactH30Zl to cause cell death was examined. p/3actH30Zl was transfected in Rat-1 cells and the X-gal reaction was developed as before. The analysis showed that the expression of pβactH30Zl did not cause cell death (Table 1). To examine if lch-1 s has any protective effect against cell death, a stable Rat-1 cell line that express lch-1 s was established. The cDNA Ich-l s was cloned into pBabepuro retroviral expression vector (Morgenstern et al. , Nucl. Acids Res. 78:3587-3596 (1990)) and transfected into Rat-1 cells. The stable transfectants were selected in puromycin and individual clones were assayed for expression of lch-1 s by Northern blot analysis. The clones that expressed Ich-l s were used for analysis and the clones that did not express lch-1 s were used as negative controls together with untransfected Rat-1 cells. When plated in non-confluent density and washed carefully, Rat-1 cells would die in serum-free medium. Under these conditions, Rat-1 cells expressing bcl-2 or crmA were resistant to death (Fig. 15). When the ability of the stable Rat-1 cell lines that express human Ich-l s was tested under serum-free conditions, it was found that they are more resistant to serum deprivation than parental Rat-1 cells and negative control transfectants not expressing Ich-l s (Fig. 15). These experiments suggest that Ich-l s may have the ability to prevent cell death. Since Ich-l s may prevent cell death by inhibiting Ich-l L , the inventors examined whether Rat-1 cells express lch-1. Using mouse lch-1 cDNA as a probe, an mRNA species predictive of the lch-1 transcript was detected in Rat-1 cells under low stringency conditions. Discussion The isolation and characterization of lch-1, a mammalian gene belonging to the cell death gene family oiIce/ced-3, has been described. Two distinct 7cΛ-7 mRNA species have been identified (lch-1 L and lch-1 s ). These two cDNAs differ in both 5' regions around translation initiation and in the middle region. The difference in the middle is the result of alternative use of two different 5' splicing donor sites. The lch-1 gene is expressed at low levels in both embryonic and adult tissues tested. Ich-l s is expressed at higher levels than Ich-1 L in embryonic heart and brain. The converse is true in embryonic lung, liver and kidney. The expression of both 7CE and lch-1 can be detected in THP-1 cells and DO 11.10 cells. The expression of both 7CE and lch-1 L appear to increase in dying cells under serum deprived conditions. Overexpression of lch-1 L in rat fibroblast cells caused programmed cell death. This suggests that lch-1 is also a programmed cell death gene. Overexpression of lch-1 s did not cause cell death. Stable expression of Ich-l s prevented Rat-1 cell death induced by serum deprivation. The collective results show that lch-1 encodes protein products that regulate cell death positively and negatively. The mouse nedd-2 gene was originally isolated by Kumar et al. (Biochem. & Biophy. Res. Comm. 785: 1155-1161 (1992)). The nedd-2 gene was identified as having a transcript of 3.7 kb that is abundantly expressed in embryonic day 10 mouse brain and almost undetectable in adult brain. The nedd-2 cDNA isolated contained an open reading frame of 171 amino acids and long 5' and 3' untranslated regions with stop codons in all reading frames. The 171-amino-acid open reading frame is homologous to PlO subunit oi ICE and the C-terminal part of ced-3 protein (Yuan, J., et al , Cell 75:641-752 (1993)). In the Northern blot analysis described herein, the lch-1 expression in human fetal brain is not high compared to other tissues tested (heart, lung, liver and kidney). Part of the difference could be explained by the different developmental stages tested: mouse E10 versus human 20-26 week old fetuses. However, lch-1 expression can be detected in human adult tissues. In the studies herein, amplification of the 5' untranslated regions of the mouse nedd-2 cDNA that Kumar reported was not achieved. It is possible that the 5' untranslated region in the Kumar clone was a product of incompletely processed nedd-2 mRNA. Both lch-1 mRNAs are about 4 kb; since the cDNA clones described herein are 2.5 kb and 2.2 kb for Ich-1 L and Ich-l s , respectively, these cDNAs are incomplete. However, since they are fully functional in the assay reported herein, the complete coding regions should be encoded in these two cDNAs. lch-1 is a new member of the ICE/ced-3 family of cell death genes. Thus, unlike C. elegans, mammals must have multiple members oiICE/ced-3. lch-1 is even slightly more homologous to ced-3 protein than 7CE. The cell death induced by overexpression oi lch-1 was poorly inhibited by crmA. This result is similar to that with ced-3 (Miura, M., et al , Cell 75:653-660 (1993)). The two amino acid residues of ced-3 protein that are conserved in lch-1 but not in 7CE were mutagenized. Results showed that T352A completely eliminated the ability of lch-1 to cause cell death, despite the fact that the corresponding amino acid in 7CE is a Ser, while F212L caused a reduction of the cell killing activity. These data also suggest that lch-1 may be mechanistically more similar to ced-3 than 7CE, and 7CAV7 and 7CE may have evolved independently from ced-3. The overexpression oi ICE and lch-1 can kill Rat-1 cells and HeLa cells effectively but NG108 cells only moderately. The possibility that the activity of β-actin promoter is lower in NG108 cells cannot be ruled out. However, an interesting possibility is that NG108 cells express a higher level of 7CE and lch-1 inhibitors. COS cells are completely resistant to the cell killing activity of 7CE and lch-1. COS cells may lack either the activator or the substrates of 7CE and lch-1. This result also suggests that the cytotoxic effects of 7CE and lch-1 have certain specificity and are unlikely to be caused by random cleavage activities of proteases. lch-1 can make a protein product that either prevents or causes cell death depending on how the mRNA is processed. Similar regulation has been observed with bcl-x, a bcl-2 related gene (Boise et al , 1993). The bcl-x transcripts can also be processed in two different ways: the larger mRNA, bcl-x L , encodes a bcl-2 related protein product that can inhibit cell death induced by growth factor withdrawal when overexpressed in an IL-3-dependent cell line. Alternative splicing oi bcl-x transcripts can generate another smaller transcript. bcl-x s , encodes an internal truncated version of bcl-x protein that inhibits the ability of bcl-2 to enhance the survival of growth factor-deprived cells. Control of the RNA splicing could prove to be an important differential regulatory check point in programmed cell death. How does 7cΛ-7 s act to prevent cell death? It could act either by inactivating the activator of cell death or by directly inactivating lch-1 L . Since Rat-1 cells appear to express lch-1, these two possibilities cannot be distinguished at present. In the transient transfection assay, the expression of Ich-l L -lacZ fusion gene and the 7ce-lacZ fusion gene kill the stable lch-1 s expressing cells as efficiently as the control Rat-1 cells (L. Wang, unpublished data). Thus, unlike crmA or bcl-2, the inhibition of cell death by Ich-l s may be highly dosage-dependent. This is probably why the expression of Ich-l s provided only partial protection to the cell death of Rat-1 cells induced by serum deprivation: only those cells expressing high levels of lch-l s are protected. crmA has the ability to suppress cell death induced by overexpression of Ich-1 L . The amino acid sequence of crmA protein is homologous to the members of the seφin and superfamily (Pickup et al , 1986), which usually inhibit serine proteases by acting as pseudosubstrates. The nature of interaction of 7CE and crmA protein is not fully understood but it is likely to be similar to the interaction of other seφin and serine proteases. The inhibition oi ICE family members by crmA may depend upon both the affinity and relative concentration of 7CEs and crm- protein. The fact that crmA can suppress a certain percentage of cell deaths induced by overexpression of the lch-1 L suggests that crmA and lch-1 can bind to each other although their affinity may be low. It is possible that when lch-1 concentration is lower, crmA may be able to suppress a much larger percentage of cell death induced by 7ctV7. Microinjection of crmA expression construct can effectively suppress the death of dorsal root ganglia neurons induced by nerve growth factor deprivation (Gagliardini, V., et al , Science 265:826-828 (1994)). One or more lCEIced-3 family members may be responsible for neuronal cell death. When crmA expression construct is microinjected into neurons, the transient concentration of cr -4 protein may be very high. Thus, it is possible that crmA may be able to suppress multiple members of ICEIced-3 family under such conditions despite the fact that their affinity to crmA is not very high. Since the expression of lch-1 and 7ce can be detected in the same cells, the results described herein suggest that multiple members oilce/ced-3 family may contribute to cell death induced by a single signal. There are three possible ways that 7ce and lch-1 may act to cause cell death (Figure 16). First, lch-1 may activate Ice, directly or indirectly, to cause cell death. Second, 7CE may inactivate lch-1, directly or indirectly, to cause cell death. Third, ICE and lch-1 may act in parallel to cause cell death. In the first scenario, the inhibitor of 7CE should inhibit cell death induced by lch-1. In the second scenario, the inhibitor oi lch-1 should inhibit the cell death induced by ICE. To test this hypothesis, specific inhibitors for each member of 7CH are necessary. For the reasons discussed above, it seems likely that crmA can inhibit other members of ICE/ ced-3 family as well. These models can be tested directly by "knock-out" mutant mice in which a specific member of the ICE/ced-3 family is mutated. Experimental Procedures Cloning and Construction of Plasmids The mouse nedd-2 cDNA was isolated using embryonic mouse brain cDNA and the primer pairs specific for the 5' and 3' untranslated regions and the coding region. Primers neddlll (5'-CAACCCTGTAACTCTTGATT-3') and nedd2l2 (5'-ACCTCTTTGGAGCTACCAGAA-3') were used for amplifying the 5' untranslated region. Primers nedd.213 (5'-CCAGATCTATGCTAACTGTCCAAGTCTA-3') and nedd2l4 (5ΑAGAGCTCCTCCAACAGCAGGAATAGCA-3') were used for amplifying the nedd-2 coding region. Primer nedd2/5 (AGAAGCACTTGTCTCTGCTC) and nedd2/6 (5'TTGGCACCTGATGGCAATAC-3') were used for amplifying the 3' untranslated region. 0.5 kb PCR product of nedd-2 coding region was cloned into pBluescript plasmid vector to be used as a probe (Stratagene). A human fetal brain cDNA library (Stratagene) was screened with murine nedd-2 cDNA probe at low stringency. The filters were hybridized in 5x SSPE, 30% formamide, lx Denhardt's solution, 1 % SDS at 42°C overnight and washed in lx SSPE and 0.5% SDS, twice at room temperature and twice at 45 °C (20 min). The human lch-1 s (pBSH30) was isolated from the positive clones using a BamHl-SaR fragment of the murine nedd-2 cDNA, a 76 bp fragment which contains the 61 bp intron, as a probe under the same hybridization and washing conditions described above. The phage clones (pBSH37 for Ich-1 L , pBSH30 for lch-1 __) were excised in vivo to obtain plasmids by an in vivo excision protocol (Stratagene). To construct expression constructs, PCR was performed using synthetic primers. HI (5'-GATATCCGCACAAGGAGCTGA-3') and H2 (5'-CTATAGGTGGGAGGGTGTCC-3') were used for lch-1 L construction. H3 (5'-GATATCCAGAGGGAGGGAACGAT-3'), corresponding to sequences in the 5' region of Ich-l s cDNA and H4 (5'-GATATCAGAGCAAGAGAGGCGGT-3'), corresponding to the sequences in the 3' region of the first open reading frame (ORF) of Ich-l s were used for the first ORF of lch-1 s construction. H3 and H5 (5'-GATATCGTGGGAGGGTGTCCT-3'), corresponding to the sequences in the 3' region of the second ORF of Ich-l s were used for the second ORF of Ich-l s construction. pBSH37 and pBSH30 were used as templates where appropriate. The three PCR products were inserted into the EcoRV site of pBluescript II, and the inserts were isolated by digestion with Smal and Kpnl and cloned into Smal-Kpnl sites of BSLacZ (Miura, M. , et al , Cell 75:653- 660 (1993)). NotI linkers were added to the Kpnl site by digesting with Kpnl, blunt-ending by T4 polymerase and ligating in the presence of excess NotI linker. These constructs, BSh37Z, BSh30Zl and BSh30Z2, were digested with NotI and individually cloned into pβactstneoB (which uses chicken /3-actin promoter) (Miyawaki, A. , et al. , Neuron 5: 11-18 (1990)). The final plasmids were designated p/3actH37Z, pjSactH30Zl and p/3actH30Z2, respectively. pBabeH30 plasmid, used for establishing stable Rat-1 cell line carrying Ich-l s , was constructed by inserting the full length Ich-l s cDΝA into the Sail site of pBabe/puro vector (Morgenstern, J. P., et al., Nucl. Acids Res. 78:3587-3596 (1990)). To mutagenize Cys 303 to a Ser residue in the active domain of lch-1 L , Ala 352 to a Thr residue in the PlO subunit of Ich-1 L and Leu 212 to a Phe residue in the P20 subunit of Ich-1 L , primers containing mutant sites were synthesized as follows: HM1 5'-ATCCAGGCCTCTAGAGGAGAT-3' HM2 5'-ATCTCCTCTAGAGGCCTGGAT-3' HM3 5'-TGCGGCTATACGTGCCTCAAA-3' HM4 5'-TTTGAGGCACGTATAGCCGCA-3' HM5 5'-CACAGTACTTTCGTCACCCT-3' HM6 5'-AGGGTGACGAAAGTACTGTG-3' (HM1 is corresponding with HM2, HM3 is corresponding with HM4, HM5 is corresponding with HM6). PCRs were performed in two steps. To make the Cys 303 to Ser mutation, in the first round of PCR, the fragments from the N-terminal to mutation site of lch-1 L and from the mutant site to C-terminal of lch-1 L were synthesized using two primer pairs, T3 and HM 1 , HM2 and T7, and PBSH37 as a template. In the second round PCR, the two PCR fragments generated in the first reaction were used as templates and T7 and T3 were used as primers. Two such rounds of PCR generated a full length lch-l L mutant. The other two mutations were generated in similar way using T3 and HM3, HM4 and T7 for Ala 352 to Thr mutation, and T3 and HM5, HM6 and T7 for Leu 212 to Phe mutation as primers for first PCR. The PCR products were inserted into the EcoRV site of pBluescript II and sequenced. The mutant cDNA inserts were cloned into expression vectors as described above. The mutated clones were designated p/3actH37ZCS, pj3actH37ZAT and p/3actH37LF. Cell Culture and Functional Studies All cells were maintained in Dulbecco's modified Eagle's medium (DM EM) supplemented with 10% fetal calf serum (FCS). The day before transfection, cells were seeded at a density of about 2.5xl0 5 in each of the 6- well dishes. For each well, 0.7-lμg oi LacZ chimeric construct and 10μg of lipofectamine reagent were used according to a protocol from GIBCO BRL (Gaithersburg, MD). The cells were incubated for 3 hr in serum-free medium containing DNA and lipofectamine. Then an equal volume of growth medium containing 20% serum was added without removing the transfection mixture and incubation was continued for 24 hr. The expression of the chimeric gene in cells in culture was detected as previously described (Miura, M., et al , Cell 75:653-660 (1993)). To establish Rat-1 cell lines overexpressing Ich-l s , pBabeH30 was transfected into Rat-1 cells using lipofectamine mediated gene transfer. Resistant cells were selected using 3μg/ml puromycin for about 10 days. Cells were assayed for expression of lch-1 s by Northern blot analysis. To examine whether Ich-l s can render Rat-1 cells resistant to apoptosis in the condition of serum deprivation, the Rat-1 cells overexpressing Ich-l s , untransfected control Rat-1 cells, transfected negative control Rat-1 cells and Rat-1 cells overexpressing bcl-2 or crmA were seeded in 24-well dish at 5xl0 4 cells in 500 μl of DMEM containing 10% FCS for 24 hr, and then washed once with serum-free DMEM and transferred into 500 μl of serum-free DMEM. The cells were harvested at daily intervals and stained with 0.4% trypan blue for 5 min. at room temperature. The numbers of dead and living cells were counted using a haemocytometer. RNA Analysis The Multiple Tissue Northern (MTN) blots membrane of human fetal and adult tissues (CLONTECH) were probed using human lch-1 L cDNA or the intron of lch-1 s cDNA (for fetal tissue) under conditions of 5x SSPE, lOx Denhardt's solution, 50% formamide, 2% SDS, lOOμg/ml salmon sperm DNA at 42°c for overnight. The blots were washed twice in 2x SSPE, 0.05% SDS at room temperature and twice in 0. lx SSPE, 0.1 % SDS for 20 min. at 50°c for 20 min each. In Vitro Transcription and Translation of Ice-ced 3 S homolog To determine which open reading frame of Ich-l s homolog was expressed, pBluescript plasmid containing Ich-l s (pBSH30) was linearized at the 3' multiple cloning site with Xhol, purified, and transcribed with T3 RNA polymerase for 2 hr at 37 °c using a protocol from Stratagene. The plasmid was also linearized at the 5' multiple cloning site with NotI, purified, and transcribed with T7 polymerase as an antisense control. The resulting runoff transcripts were extracted with phenol-chloroform and ethanol precipitated. In vitro translation was performed with rabbit reticulocyte lysate (Promega) in the presence of 35 S-methionine for 1 hr. at 30°C. 5μl lysate was mixed with equal volume of 2xSDS gel loading buffer and subjected to SDS- polyacrylamide gel electrophoresis (12%). The gel was dried and exposed to X-ray film. TABLE 1 Expression cassettes COS HeLa NG108-15 Rat-1 Rat-l/bcl-2 Rat-l/crmA pactb / Sgal ' • 1.3 + 0.1(983) 2.9±0.2(1020) 4.2±0.2(1535) 2.9±0.2(1470) 3.4±0.2(1446) 3.7 + 0.1(1459) pβactMlOZ • 93.9±0.3(1003) 80.2±0.5(1545) 94.2+ 1.1(978) 28.8 + 0.5(691) 45.8± 1.6(233) 11.0±0.2(1080) p/?actH37Z 8.3±0.9(1053) 91.4±0.2(1076) 68.7± 1.5(1605) 92.1 ±0.3(1079) 21.5 + 3.2(1335) 80.7 + 0.9(1010) pj3actH37ZCS ND 5.6±0.1(1039) 5.9±0.9(707) 4.1 ±0.2(1477) ND ND p/3actH37ZAT ND 8.2±0.7(435) 5.2±0.2(640) 5.4±0.3(1356) ND ND p/JactH37ZLF ND 75.8±2.2(1404) 39.9±3.8(1193) 77.4±0.4(1704) ND ND o p/3actH30Zl 1.3 ±0.2(676) 0.0±0.0(40) 0.0±0.0(61) 1.8+0.4(785) ND ND Table 1. The constructs as described in the text were transiently transfected into Rat-1 cells, Rat-1 cells expressing human bc/-2, Rat-1 cells expressing cowpox virus crmA gene, HeLa cells, NG108- 15 cells and COS cells. Cells were fixed lightly 24 hr after transfection and stained with X-Gal for 3 hr. The data (mean ±S EM) shown are the percentage of round blue cells among total number of blue cells counted. The numbers in the parentheses are the number of blue cells counted. The data were collected from at least three independent experiments. ND = not determined.

Claims: What Is Claimed Is: 1. A method of preventing programmed cell death in vertebrates comprising the step of inhibiting the enzymatic activity of interleukin-l converting enzyme (ICE). 2. The method of claim 1 , wherein said enzymatic activity is inhibited by an /CE-specific antiprotease. 3. The method of claim 2, wherein said antiprotease is crmA. 4. A method of promoting programmed death in vertebrate cells by increasing the enzymatic activity of ICE in said cells. 5. The method of claim 4, wherein said vertebrate cells are cancer cells. 6. The method of claim 5, wherein said cancer cells overexpress the oncogene bcl-2. 7. A substantially pure gene which is preferentially expressed in thymus and placental cells and which encodes a protein causing programmed cell death. 8. The gene of claim 7, wherein said protein has the amino acid sequence shown in Figure 6. 9. The gene of claim 8, wherein said gene has the cDNA sequence shown in Figure 6. 10. An expression vector having the gene of either claim 8 or claim 9. 11. A host cell transformed with the vector of claim 10. 12. A substantially pure protein wherein said protein is preferentially expressed in thymus or placental cells and which causes the death of said cells. 13. The protein of claim 12, wherein said protein has the amino acid sequence of mICE2 as shown in Figure 6. 14. A functional derivative of the protein of claim 13. 15. A method of promoting programmed cell death in thymus or placental cells comprising the step of increasing the activity of the protein of claim 7. 16. A substantially pure DNA molecule comprising the cDNA sequence of human lch-1 as shown in Figure 8. 17. An expression vector having the DNA of claim 16. 18. A host cell transformed with the vector of claim 17. 19. A substantially pure protein comprising the amino acid sequence of human lch-1 as shown in Figure 8. 20. A functional derivative of the protein of claim 19. 21. A substantially pure DNA molecule comprising the cDNA sequence of human lce-4 homolog as shown in Figure 16. 22. An expression vector having the DNA of claim 21. 23. A host cell transformed with the vector of claim 22. 24. A substantially pure protein comprising the amino acid sequence of human lce-4 homolog as shown in Figure 16. 25. A functional derivative of the protein of claim 24.

Inventors: MIURA MASAYUKI, YUAN JUNYING, MIURA, MASAYUKI, YUAN, JUNYING
Applicants: GEN HOSPITAL CORP, THE GENERAL HOSPITAL CORPORATION
Application year: 1994
Main classification: A61K37
IPC codes: A61K38, A61P43, C07H21, C07K14, C12N1, C12N5, C12N9, C12N15, C12P21, F02B75